You need to explain a little further. Why can't you install the update? You can only update to iOS 5.1.1 if you have the original iPad.
Connect your iPad to your computer and launch iTunes. Click on the iPad name under devices on the left. Click on the Summary Tab in the iTunes window on the right and click on Check for Update.
iOS 5: Updating your device to iOS 5 or Later
If you are currently running an iOS lower than 5.0, connect to your computer & click on the "Update your device using iTunes".
Tip - 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.