Editing of an existing object should be done in repository layer or in service?

You should leave the responsibility of mutating an object to that same object and use the repository to retrieve this object.

Example situation:

class User { private int debt; // debt in cents private string name; // getters public void makePayment(int cents){ debt -= cents; } } class UserRepository { public User GetUserByName(string name){ // Get appropriate user from database } } 

Usage (Jack pays 10€):

userRepository.GetUserByName("Jack").makePayment(1000); 

Keep in mind that this is just an example approach. There is not one set way in programming to achieve something, you could very well do this entirely different.