how do I find out what Apple account it's linked to and transfer it to my account? on my MacBook Pro

How do I find out what Apple account it's linked to and transfer it to my account? trying to download music to MacBook but keep getting this response. "You must transfer this device to your Apple Account to play content saved offline. You will be unable to transfer this device again for 90 days."

How to download music on my MacBook? I have owned it for well over 90 days and had this problem since day one. anyone know how to fix or figure out what acct its still on??