Have you got the latest version of iTunes on the computer, and is it the computer that you normally sync to ? If yes then with the iPad connected to it and selected on the left-hand side of iTunes, select the Summary tab on the right-hand side and click the Check For Updates button. If it is not the computer that you normally sync to and it doesn't have all your content on it, then you risk losing your iPad's content if you update via it.
Updating to iOS 5 : http://support.apple.com/kb/HT4972
If you are already on iOS 5+ on your iPad (Settings > General > About > Version) then you should be able to update to 5.1 directly on the iPad via Settings > General > Software Update.