in reference to: (view on Google Sidewiki)
How to restore or update your iPod/iPhone/iPad:
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.