What version of iTunes is on your computer? For an iPad 2, you should use at least iTunes 10.6.3.
iTunes 10.6.3 download http://support.apple.com/kb/DL1575
It's for your Mac or PC, not your iPad.
If you have an iPad 1, the max iOS is 5.1.1. For newer iPads, the current iOS is 6.1.3. 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
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 an iPad (except iPad 1) to iOS 6.x, the next update can be installed via wifi (i.e., not connected to your computer).
Tip 2 - If you're updating via wifi, place your iPad close to your router to preclude getting a corrupted download.
Quit and start all over again. Eject your iPad, quit iTunes, reboot your computer and then try again. It wouldn't hurt to restart the iPad either.
You might want to turn off your firewall and disable your antivirus software as well. You will be downloading a fairly large file and it can take a while.