Download the Windows 7 ISO.
Burn the Windows 7 ISO image to a DVD, and then copy and whole image to a storage location that you wish to run the upgrade from (can be either any directory/folder on any partition/drive on the machine running the pre-release build, or a bootable USB/FireWire flash drive).
Alternatively, it’s possible to directly extract the content of the ISO to a desired folder using file extraction tool such as WinRAR.
Browse to the sources directory.
Open the file cversion.ini in a text editor such as Notepad.
Modify the MinClient build number to a value lower than the down-level build. For example, change 7100 to 7000.
Save the file in original place and original name.
Start the setup process of Windows 7 as per normal from the modified copy of the installation files, and the version check will be bypassed