How is your internet connection? If your connection isn't strong enough, the update may not show. My iPad 2 is updating right now, without any problems.
Have you tried connecting your iPad to iTunes on your computer and using the update function? If the update function doesn't work, try the restore option. This will erase your iPad and install the latest operating system. Before starting, backup your iPad and use the backup to set up your iPad when the restore is finished. After set up is finished, you'll have an updated iPad with everything intact.