Since GM Candidate and the final version of OS X El Capitan have a different build number: how to upgrade? Just by installing the whole image from the Mac App Store?
Click on download, you'll be prompted that its already installed. Override that and install it anyway. Once done you'll be on the latest release version. Just done it myself and its gone through no problem.
Click on download, you'll be prompted that its already installed. Override that and install it anyway. Once done you'll be on the latest release version. Just done it myself and its gone through no problem.