could you please help me to create test class for the below code, as mock test response need to create here, and iam new for this.
here the intraction between web application and salesforce is happning,
getting input from web application to update the record having valid recordID. 
@RestResource(urlMapping='/LeadRestUpdate/*')

Global with sharing class  NS_WS_Lead_update {
    @HttpPost
    Global static String doUpdate()
    {
        RestRequest req = RestContext.request;
        RestResponse res = Restcontext.response;
        System.debug('Params: ' + req.params);
        //String temp = req.requestBody.toString();
        String leadid = req.params.get('lead_id');
        Lead ll = [SELECT Id, FirstName,LastName, Phone, email, company FROM Lead WHERE Id = :leadid];
        if (req.params.get('step') == 'step1' || req.params.get('step') == 'step2' )
        {
            ll.FirstName = req.params.get('first_name');
            ll.LastName = req.params.get('last_name');
            ll.phone = req.params.get('phone');
            ll.Email = req.params.get('email');
            ll.Company = req.params.get('company_name');
            ll.Company_Registration_Number__c = req.params.get('registration_number');
            ll.Company_Status__c = req.params.get('company_status');
            ll.CompanyType__c = req.params.get('company_type');
            if(req.params.get('incorporated_date') != null && req.params.get('incorporated_date') != '' ) {
                ll.Incorporation_Date__c = Date.valueOf(req.params.get('incorporated_date'));
            }
            ll.Street = req.params.get('street');
            ll.City = req.params.get('city');
            ll.State = req.params.get('state');
            ll.Country = req.params.get('country');
            ll.PostalCode = req.params.get('zip');
            
        }
        if (req.params.get('step') == 'step2')
        {
            ll.Industry = req.params.get('industry');
            if(req.params.get('take_card_payment') == 'true')
               {
                ll.Do_you_take_Card_transactions__c = Boolean.valueOf( req.params.get('take_card_payment'));
                ll.Average_monthly_card_takings__c = req.params.get('cardtakings_amount');
                ll.Average_monthly_takings_4_200__c = Boolean.valueOf( req.params.get('cardtakings'));
                ll.Average_monthly_transactions_10__c = Boolean.valueOf( req.params.get('cardtransaction10andmore'));
            }
        }
        if (req.params.get('step') == 'step3')
        {
            Double amount_requested =  Double.valueOf(req.params.get('amount_requested'));
            ll.Amount_Requested__c = amount_requested;
            ll.Term_Requested__c  = Double.valueOf(req.params.get ('loan_term'));
            ll.Purpose_of_Facility__c = req.params.get('puropose_of_funding');
        }
        if (req.params.get('step') == 'step4')     
        {
            //ll.Web_to_Lead__c = Boolean.valueOf(req.params.get('web_to_lead'));
            ll.Residential_Address_City__c = req.params.get('residential_city');
            ll.Residential_Address_Country__c = req.params.get('residential_country');
            ll.Residential_Address_State_Province__c = req.params.get('residential_state');
            ll.Residential_Address_Street__c = req.params.get('residential_street');
            ll.Residential_Address_Zip_Postal_Code__c = req.params.get ('residential_zip');
            if (req.params.get('product_interested') != 'bca') {
                ll.Home_Owner__c = Boolean.valueOf(req.params.get('own_a_property'));
                ll.Ownership_is_of_Residential_Address__c = Boolean.valueOf(req.params.get('same_as_residency'));
                ll.Property_Address_City__c = req.params.get('property_city');
                ll.Property_Address_Country__c = req.params.get('property_country');
                ll.Property_Address_State_Province__c = req.params.get('property_state');
                ll.Property_Address_Street__c = req.params.get('property_street');
                ll.Property_Address_Zip_Postal_Code__c = req.params.get('property_zip');
            }
            if (req.params.get('allow_credit_search') == 'true')
                ll.Allow_searches_on_all_directors__c = Boolean.valueOf( req.params.get('allow_credit_search'));  
            else
                ll.Allow_searches_on_all_directors__c = false;
            if(req.params.get('allow_engaging_card_provider')!=null)
               {
                ll.Authorization_to_contact_card_provider__c = Boolean.valueOf( req.params.get('allow_engaging_card_provider'));
            }
            if(req.params.get('take_card_payment') == 'true')
               {
                ll.Do_you_take_Card_transactions__c = Boolean.valueOf( req.params.get('take_card_payment'));
                ll.Average_monthly_card_takings__c = req.params.get('cardtakings_amount');
                ll.Average_monthly_takings_4_200__c = Boolean.valueOf( req.params.get('cardtakings'));
                ll.Average_monthly_transactions_10__c = Boolean.valueOf( req.params.get('cardtransaction10andmore'));
            }
        }
        update ll;
        string messagesent='Thank You. Salesforce has Successfully Updated'+ll.id+req.params.get('step');
        return messagesent;
    }
}