Generally speaking, all you have to do is install Norton in your computer, click renew at the bottom of the page and insert your product activation key. The key will be in a Symantec folder in My Documents in the computers that are activated, or available on your My Norton Account. If you have a 2006 and newer, you can upgrade for free to the newest version on all your machines.
Downloads for the most recent versions can be had here.
For assistance if required, the Norton forums are here.