If you are using iOS 4 (or earlier) there is no Software Update in Settings. That feature was first added in iOS 5.
To update you will have to connect your iPad to iTunes on a computer (preferably the one you sync with and backup to).
See here: http://support.apple.com/kb/HT4972
Note that the iPad 1 can only be updated to iOS 5.1.1.
Updating from the device itself is a feature of iOS 5 and above. You must use iTunes on your computer. Also understand that if you have an original iPad, you will be able to upgrade only to iOS 5.1.1, the highest version compatible with that device.
Follow the instructions in the section Update your device using iTunes.