Apple store.

So I got an Iphone 7+ and I an application had to make an update , youtube , so I went to apple store clicked on update and it gave me the apple id from the person who had the phone before me , it has been a long time that I have it so I don't know why. The problem is that to do any update I have to type his mail and password ( which I don't have ). I can't even change the Apple id from the app store. What should I do ?