How to restore or update your iPod/iPhone:After downloading the desired firmware below, you can shift-click (PC) or option-click (Mac) the Restore or Update buttons in iTunes. A file dialog will open and let you choose the downloaded ipsw-file. Note: Your browser may change the ipsw-file into a zip-file. In that case, just rename it to end in .ipsw and iTunes will accept it.
If you don't know what iPod or specific iPod generation you got, click here.
http://www.felixbruns.de/iPod/firmware/