In the app store, the default apple id to sign in is wrong
In the app store, when I try to update an App, the default apple id that comes up to Sign In with is wrong (it is my wife's). It is greyed out so I can't change it.
I have tried several things suggested, such as having signing out and back in (under the menu Store > Sign out). I went into the system preferences, into users & groups and I'm listed as the the user. I logged in and out of iTunes.
It still comes up wrong. We both recently upgraded to Yosemite, my wife doing it first on her own Mac. Hers works fine. Significant to note, she originally used this Mac for a brief time.
Why is her Apple ID coming up on my Mac when I go to upgrade now?
To clarify:
In the sign in window that comes up when I choose to upgrade, I am asked:
"Sign in to download from the App Store."
Her email is listed and is unchangeable, and just asks for the password.
How can I correct this? Any other ideas?
MacBook Pro, OS X Yosemite (10.10.4)