On my 2009 MacBook, my friend signed into his Apple ID to check it. Now, I can't sign back into mine to download things.
Does anyone know how to get my Apple ID to sign me back in?
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In the Mac App Store, go to the Store menu -> Sign Out. This will allow you to sign out of the store.
Note, if he made any purchases during that time, downloaded any apps and/or accepted the updates for the iWork/iLife applications, they may b tied to their AppleID and won't process under yours.