Being asked to log into a different account on Mac App Store?
To make the AppStore forget the old credentials with which an app was downloaded, remove the _MASReceipt directory in the app itself.

To do this:


Open Finder and navigate to Applications.
Ctrl+Click the application and choose "Show Package Contents".
Expand the Contents directory and click the _MASReceipt directory to select it.
Type Command+Delete to delete the directory permanently---you will be prompted for your credentials since this is a protected file.


Quit and restart AppStore, then find the application.  The button should now say "Free" or "Install" instead of "Update".  Clicking it will update your app to the latest version, doing so as the currently logged-in Apple user.

This can also be a temporary issue and it can clear up over time once the servers recover from some overload conditions.