You can google it and it comes up ten places.. including lifehacker.
basically you just use method to delete the file that chooses the version. Then windows installer just asks you which version you want to install. (HOME/PRO/ULTIMATE)
It's pretty simple.
I have done it.
Very much like the hack you use to install WHS2011 to a smaller than 160GB drive.
It's simple enough: if you have an .ISO file use an appropriate .ISO imaging program to delete the file from it, then save the image and burn it. If you have the DVD you can use the same program to rip the DVD to an .ISO image, then delete the file, and reburn it.
Or delete and install from USB works fine too.
To restart the clock,
Click Start, All Programs, Accessories. Right-click Command Prompt and choose Run As Administrator. If necessary, enter your administrator password.
Type "slmgr -rearm" and press the Enter key.
Note we are talking about the same thing.. but there is many ways to do it.