Connect your iPod, and it should open a screen in iTunes showing a picture of your iPod, with a bar at the bottom representing how much music/videos/apps, etc are on the iPod. There's a button in the middle of the page that says "Check For Updates", clicking that should prompt you to buy 2.2.1.
Alternately, click this link, which will open in iTunes:
https://phobos.apple.com/WebObjects/MZFinance.woa/wa/touchLandingPage