[How do I release the land that I own as a premium account to change to a basic account?]

I am trying to change my account from a premium account to a basic account, but when I try to do that I get a message stating that I ned to release the land I own before changing my account type, as basic accounts are not allowed to own land.  The only land i "own" is the 512 meters squared I am alloted as a premium member.