The software update feature was only added in iOS 5. To update you'll need to connect you iPad to a computer running iTunes and update through there by plowing the prompts.
Follow the link, and scroll down to the section titled: Update your device using iTunes
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
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).