Source:http://windowsitpro.com/

Q. How do I upgrade from the Windows 7 beta to the Release Candidate?

A. There’s no officially supported way to upgrade from the beta of Windows 7 (build 7000) to the Windows 7 Release Candidate (build 7100). Microsoft developers wanted to focus on the common end user upgrade experience, which will be from Windows Vista to Windows 7, and didn’t want to waste cycles on getting a perfect upgrade experience from a beta. It is also expected that there will be no direct upgrade from the Release Candidate to the final release.

There is, however, a way you can do an in-place upgrade. This method will probably also work for an upgrade from RC to final.

• Copy the contents of the Windows 7 RC to your local hard disk.
• Access the Source directory within the copied Windows 7 RC location.
• Open cversion.ini in Notepad.
• Change the MinClient value from 7100 to 7000, as shown here:

[HostBuild]

MinClient=7000

MinServer=7000

• Save the file.
• You’ll now be able to perform an in-place upgrade from the beta to the RC.