What you mean by format? Do you want to Reinstal OS or just Memory Format?
Well... I have an HTC Touch Dual with MW 6.1 so it will kinda be the same:
Go to:
START - SETTINGS - SYSTEM and then from there look for "Clear Storage" icon and click it and then follow the steps.
If you want to Reinstal the OS or instal a newer version, try Google for the OS Instalation file, then connect your phone via USB to your PC and start the Setup. Follow the steps and you will be fine.
Hope this helped.