please help me 
how to create the test class for the class given below:

global with sharing   class OutboundCallForPartnersController
{
    public Id WhatId{get;set;}
    public String PartnerName{get;set;}
    public Id TaskId{get;set;}
    public Id TaskIdFollowup{get;set;}
    
    public Task objTask{get;set;}
    public Task objTaskFollowup{get;set;}
     
    public Task objTaskOwner{get;set;}
    public Task objTaskOwnerFollowUp{get;set;}
    
    public User objUser{get;set;}
    public User objUserFollowup{get;set;}
    public Partner_lead__c objparLead{get;set;}
    public Account objAccount{get;set;}
    
    public OutboundCallForPartnersController(ApexPages.StandardController controller) 
    {     
      // LoadTasks();
    }
    
     
   public List<SelectOption> getNextStepOptions() {
        List<SelectOption> options = new List<SelectOption>();
        options.add(new SelectOption('Not Started','Not Started'));
        options.add(new SelectOption('In Progress','In Progress'));
        options.add(new SelectOption('Completed','Completed'));
        options.add(new SelectOption('Waiting on someone else','Waiting on someone else'));         
        options.add(new SelectOption('Deferred','Deferred')); 
        return options;
    }


   public  pagereference  LoadTasks() {
   
       WhatId  = System.currentPageReference().getParameters().get('WhatId');
       PartnerName  = System.currentPageReference().getParameters().get('PName');
       
       //Approved Partner Insert
       If(PartnerName=='E')
       {          
            Task t = new Task();
            //t.Approved_Partners__c=WhatId;
            t.WhatId=WhatId;
            t.Status = 'Completed';
            t.ActivityDate=date.today();
            insert t;
            //New Task ID
            TaskId=t.Id;
              //Get the Owner ID
            objTaskOwner=[SELECT OwnerId FROM Task WHERE Id =:TaskId  limit 1];  
            //Owner NAme
            objUser=[select Name from user where Id=:objTaskOwner.OwnerId];
            //Get The Task
            objTask= [SELECT ReminderDateTime ,ActivityDate,Disposition__c,Call_Duration_In_Seconds__c,Call_End_Time__c,Call_Start_Time__c,Comment_Summary__c,CTI_Id__c,Description,Id,LastModifiedById,LastModifiedDate,OwnerId,PartnerLead__c,Priority,Status,Subject,Subject_C__c,WhatId,WhoId FROM Task WHERE Id = :TaskId and Status = 'Completed'];
           
            // Follow Up Entry 
            Task Ft = new Task();        
            //Ft.Approved_Partners__c=WhatId;
            Ft.WhatId=WhatId;        
            Ft.Status = 'Not Started';
            Ft.ActivityDate=date.today();
            insert Ft;
               //New Follow Up TASK ID
             TaskIdFollowup =Ft.Id; 
            // =[Select Id from Task where whatId=:WhatId and Status != 'Completed' order by CreatedDate desc limit 1];
            // Folloup Owner ID
            objTaskOwnerFollowup=[SELECT OwnerId FROM Task WHERE Id =:TaskIdFollowup  limit 1]; 
               //Owner Details
            objUserFollowup=[select Name from user where Id=:objTaskOwnerFollowup.OwnerId];
               //Folloup TAsk
            objTaskFollowup= [SELECT IsReminderSet,ReminderDateTime, ActivityDate,Disposition__c,Call_Duration_In_Seconds__c,Call_End_Time__c,Call_Start_Time__c,Comment_Summary__c,CTI_Id__c,Description,Id,LastModifiedById,LastModifiedDate,OwnerId,PartnerLead__c,Priority,Status,Subject,Subject_C__c,WhatId,WhoId FROM Task WHERE Id = :TaskIdFollowup and Status != 'Completed'];
            
            Account();
       }
       else If(PartnerName=='F')
       {          
            Task t = new Task();
            //t.Approved_Partners__c=WhatId;
            t.WhatId=WhatId;
            t.Status = 'Completed';
            t.ActivityDate=date.today();
            insert t;
            //New Task ID
            TaskId=t.Id;
              //Get the Owner ID
            objTaskOwner=[SELECT OwnerId FROM Task WHERE Id =:TaskId  limit 1];  
            //Owner NAme
            objUser=[select Name from user where Id=:objTaskOwner.OwnerId];
            //Get The Task
            objTask= [SELECT ReminderDateTime ,ActivityDate,Disposition__c,Call_Duration_In_Seconds__c,Call_End_Time__c,Call_Start_Time__c,Comment_Summary__c,CTI_Id__c,Description,Id,LastModifiedById,LastModifiedDate,OwnerId,PartnerLead__c,Priority,Status,Subject,Subject_C__c,WhatId,WhoId FROM Task WHERE Id = :TaskId and Status = 'Completed'];
           
            // Follow Up Entry 
            Task Ft = new Task();        
            //Ft.Approved_Partners__c=WhatId;
            Ft.WhatId=WhatId;        
            Ft.Status = 'Not Started';
            Ft.ActivityDate=date.today();
            insert Ft;
               //New Follow Up TASK ID
             TaskIdFollowup =Ft.Id; 
            // =[Select Id from Task where whatId=:WhatId and Status != 'Completed' order by CreatedDate desc limit 1];
            // Folloup Owner ID
            objTaskOwnerFollowup=[SELECT OwnerId FROM Task WHERE Id =:TaskIdFollowup  limit 1]; 
               //Owner Details
            objUserFollowup=[select Name from user where Id=:objTaskOwnerFollowup.OwnerId];
               //Folloup TAsk
            objTaskFollowup= [SELECT IsReminderSet,ReminderDateTime,ActivityDate,Disposition__c,Call_Duration_In_Seconds__c,Call_End_Time__c,Call_Start_Time__c,Comment_Summary__c,CTI_Id__c,Description,Id,LastModifiedById,LastModifiedDate,OwnerId,PartnerLead__c,Priority,Status,Subject,Subject_C__c,WhatId,WhoId FROM Task WHERE Id = :TaskIdFollowup and Status != 'Completed'];
            
            Account();
       }
       //Partner Lead Insert
       else if (PartnerName=='G')
       {
            //TAsk Entry
            Task t = new Task();
            //t.PartnerLead__c=WhatId;
            t.WhatId=WhatId;
            t.Status = 'Completed';
            t.ActivityDate=date.today();
            insert t;
               //New Task ID
            TaskId=t.Id;
            //TaskId  =[Select Id from Task where whatId=:WhatId and Status = 'Completed' order by CreatedDate desc limit 1];
            //Owner ID
            objTaskOwner=[SELECT OwnerId FROM Task WHERE Id =:TaskId  limit 1]; 
               //Owner Details
            objUser=[select Name from user where Id=:objTaskOwner.OwnerId];
               //Task Details
            objTask= [SELECT ActivityDate,Disposition__c,Call_Duration_In_Seconds__c,Call_End_Time__c,Call_Start_Time__c,Comment_Summary__c,CTI_Id__c,Description,Id,LastModifiedById,LastModifiedDate,OwnerId,PartnerLead__c,Priority,Status,Subject,Subject_C__c,WhatId,WhoId FROM Task WHERE Id = :TaskId and Status = 'Completed'];
           
            //New Followup Entry
            Task Ft = new Task();         
            Ft.WhatId=WhatId;
            //Ft.PartnerLead__c=WhatId;
            Ft.Status = 'Not Started';
            Ft.ActivityDate=date.today();
            insert Ft;         
            TaskIdFollowup =Ft.Id; //New followup task ID
            objTaskOwnerFollowup=[SELECT OwnerId FROM Task WHERE Id =:TaskIdFollowup  limit 1];  // owner ID        
            objUserFollowup=[select Name from user where Id=:objTaskOwnerFollowup.OwnerId];    //owner Details
            //FollowUp Task Details
            objTaskFollowup= [SELECT IsReminderSet,ReminderDateTime, ActivityDate,Disposition__c,Call_Duration_In_Seconds__c,Call_End_Time__c,Call_Start_Time__c,Comment_Summary__c,CTI_Id__c,Description,Id,LastModifiedById,LastModifiedDate,OwnerId,PartnerLead__c,Priority,Status,Subject,Subject_C__c,WhatId,WhoId FROM Task WHERE Id = :TaskIdFollowup and Status != 'Completed'];
            
            PartnerLead();
       }
       else if (PartnerName=='H')
       {
            //TAsk Entry
            Task t = new Task();
            //t.PartnerLead__c=WhatId;
            t.WhatId=WhatId;
            t.Status = 'Completed';
            t.ActivityDate=date.today();
            insert t;
               //New Task ID
            TaskId=t.Id;
            //TaskId  =[Select Id from Task where whatId=:WhatId and Status = 'Completed' order by CreatedDate desc limit 1];
            //Owner ID
            objTaskOwner=[SELECT OwnerId FROM Task WHERE Id =:TaskId  limit 1]; 
               //Owner Details
            objUser=[select Name from user where Id=:objTaskOwner.OwnerId];
               //Task Details
            objTask= [SELECT ActivityDate,Disposition__c,Call_Duration_In_Seconds__c,Call_End_Time__c,Call_Start_Time__c,Comment_Summary__c,CTI_Id__c,Description,Id,LastModifiedById,LastModifiedDate,OwnerId,PartnerLead__c,Priority,Status,Subject,Subject_C__c,WhatId,WhoId FROM Task WHERE Id = :TaskId and Status = 'Completed'];
           
            //New Followup Entry
            Task Ft = new Task();         
            Ft.WhatId=WhatId;
            //Ft.PartnerLead__c=WhatId;
            Ft.Status = 'Not Started';
            Ft.ActivityDate=date.today();
            insert Ft;         
            TaskIdFollowup =Ft.Id; //New followup task ID
            objTaskOwnerFollowup=[SELECT OwnerId FROM Task WHERE Id =:TaskIdFollowup  limit 1];  // owner ID        
            objUserFollowup=[select Name from user where Id=:objTaskOwnerFollowup.OwnerId];    //owner Details
            //FollowUp Task Details
            objTaskFollowup= [SELECT IsReminderSet,ReminderDateTime, ActivityDate,Disposition__c,Call_Duration_In_Seconds__c,Call_End_Time__c,Call_Start_Time__c,Comment_Summary__c,CTI_Id__c,Description,Id,LastModifiedById,LastModifiedDate,OwnerId,PartnerLead__c,Priority,Status,Subject,Subject_C__c,WhatId,WhoId FROM Task WHERE Id = :TaskIdFollowup and Status != 'Completed'];
            
            PartnerLead();
       }
       return null;
     }
         
    private void PartnerLead()
    {
       objparLead=[select Id,Phone__c,Cell_Phone__c from Partner_lead__c where Id =: WhatId];
    }
    private void Account()
    {
       objAccount=[select Id,Phone,Cell_Phone__c from Account where Id =: WhatId];
    }
    
     public void submit() {
        try {
            if(objTask.Id!=NULL) 
            {
                update objTask;               
            } 
            if(objTaskFollowup.Id!=NULL) 
            {
                update objTaskFollowup;               
            } 
            else 
            {
                ApexPages.AddMessage(new ApexPages.Message(ApexPages.Severity.ERROR,'Please Enter')); 
            }            
        } 
        catch(Exception e)
        {
            System.debug('-----Exception -----' + e);
            ApexPages.AddMessage(new ApexPages.Message(ApexPages.Severity.ERROR,'System Error : Contact System Admin')); 
        }      
    }
}

I want to change this by "Call Duration (Seconds)"

i have follow this URL https://help.salesforce.com/articleView?id=customize_rename.htm&type=0
but this only to change the Field Label not the field Name

Please Help