As all of us already know that Microsoft has made it clear that there won’t be an upgrade path from Windows 8 Release Preview or for any other test versions of Windows 8 to Windows 8 RTM.
But then there are always the enthusiasts who try to work things around and come up with something.So Mr. Denny From IT Knowledge Exchange has come up with a workaround which allows not just the Release Preview users but even those who are running the Consumer Preview or even the Developer Preview can upgrade to Windows 8 RTM.
Here’s how it works,
Firstly search for the n:\sources\cversion.ini file in the installation media,after locating the file,open it up in notepad and change the 8508 to 7100,then save and exit.
You are almost done !now you just have to proceed with the installation of Windows 8 RTM from your existing Preview version of Windows 8.Since there is no official upgrade path from the earlier version to Windows 8,this process is not supported,so try it at your own risk.