Here are the instructions. See the part about update using iTunes.
Or .....you can connect the iPad to the computer, launch iTunes. Select the iPad under the devices heading on the left, click on the Summary tab and then click on Check for Update
If you have an iPad 1, the max iOS is 5.1.1. For newer iPads, the current iOS is 6.0.1. The Settings>General>Software Update only appears if you have iOS 5.0 or higher currently installed.
iOS 5: Updating your device to iOS 5 or Later
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.