The answer to upgrade from Win 7 RC to Win 7 Final Release can be found at the below link. I also included the Q and A from the page for a quick answer to the OP's question.
http://www.microsoft.com/windows/windows-7/faq.aspx QCan I upgrade from the RC to the final version of Windows 7?
AWhen you install the final version of Windows 7, you'll need to do a clean installation. So plan on backing up your data then reinstalling your applications and restoring your data.