I had a working iphone 6s 16 and i went to update it on Apple Service centre and they updated it. But i dont know what they did to it. It is an iphone 6s from outside but the firmware is of iphone 5s and also No functions of 6s are working not even 3d touch. Now they are asking for the Bill of the phone so they could replace it (maybe) ?? Is it possible?? What should i do??