You can update iPad software in Settings (with iOS 5), or by using iTunes.
Update wirelessly on iPad: Go to Settings > General > Software Update. iPad checks for available software updates.
Update software in iTunes: iTunes checks for available software updates each time you sync iPad using iTunes or you can go tothe Summary tab and click the "Check for Updates" button.
iOS 5: Updating your device to iOS 5 or Later
How to install iOS 6
iOS: How to update your iPhone, iPad, or iPod touch
If you are currently running an iOS lower than 5.0, connect the iPad to the computer, open iTunes. Then select the iPad under the Devices heading on the left, click on the Summary tab and then click on Check for Update.
Tip - If connected to your computer, you may need to disable your firewall and anitvirus software temporarily. Then download and install the iOS update. Be sure and backup your iPad before the iOS update. After you update to iOS 6.x, the next update can be installed via wifi (i.e., not connected to your computer).