Hi,

Please find the below code where I have used the Map.keySet();

Code:
****************


trigger ConTrgToUpdateParent on Contact (after update) {

   
    Map<Id,Contact> Mp = new Map<Id,Contact>();
    List<Account> aLstToUpdate = new List<Account>();
   
    //indexed for loop
    for(integer i=0; i<trigger.new.size(); i++){
       
       
        //condition to check if the phone value is changed
        if(trigger.new[i].Phone != trigger.old[i].phone){           
           
            //Adding the Account Id's to a set
            //aIds.add(trigger.new[i].AccountId);
           // System.debug('Mp'+Mp.size()+'----'+Mp);
            System.debug('trigger.old[i].AccountId : '+trigger.old[i].AccountId);
            //System.debug('trigger.new[i] : '+trigger.new[i]);
           
            Mp.put(trigger.old[i].AccountId,trigger.new[i]);           
           
        }
        //Querying the Account records where the contact phone is changed..
        for(Account acc :[Select Id,phone FROM Account WHERE Id =: Mp.keySet()]){          
           
           
            //updating the account phone with the contact changed phone value..
            system.debug('Mp.get(acc.Id) :'+Mp.get(acc.Id));
            acc.phone = Mp.get(acc.Id).phone;
           
            //adding the account with changed phone to a list..
            //update acc;//
            aLstToUpdate.add(acc);
        }
        //updating the account list with new phone values
        update aLstToUpdate;
       
    }
}

Thanks,
Vijay