You always have to update via restore from the last beta version of iOS to the final, public release.
If you were a developer, you would download the iOS 6 GM from the developer portal.. Since it sounds like you are not a developer, you will need to find the appropriate ipsw for your device on the Internet, download it, and then hold down the [option] button and click restore in iTunes, point it to the file you downloaded, and you will be updated to the public (GM) release.