Burn the ISO and then copy the image to a location you wish to run the upgrade from (a bootable flash drive, a directory on any partition on the machine running the pre-release build).
- Browse to the sources directory.
- Open the file cversion.ini in a text editor like Notepad.
- Modify the MinClient build number to a value lower than the down-level build. For example, change 7100 to 7000 (pictured below).
- Save the file in place with the same name.
Run setup like you would normally from this modified copy of the image and the version check will be bypassed.