I am trying to update software on a Macbook. Before doing so, apparently I have to "Sign in to download from the App Store." It provides me with a form asking for an Apple ID and password to do so.
However, the Apple ID field already has an Apple ID in it, which is not the one I wish to use, and the field is disabled, so I can't change it.
Why is the field disabled? How do I enable it so I can change the Apple ID to sign in with?