Changing Apple ID

HI I just updated my iPad but it changed my apple id username to something I didn't have it on before. I don't know the password so I can't sign out or reset my iPad as that requires the password. When I go on the apple website,nthough, it accepts my old one but it doesn't change it on settings, so I'm stuck here. Should I go to an Apple Store and ask them to change it or is there a way I can do it myself without knowing the password for the username it set it to?