With iOS 4 you can only update the iPad via your computer's iTunes, as described on the page that you posted from.
Connect the iPad to your computer's iTunes and copy any purchases off the iPad to your computer via File > Devices > Transfer Purchases. You may also want to copy photos and any important documents off the iPad as well e.g. via the file sharing section at the bottom of the device's apps tab when connected to iTunes, via wifi, email, dropbox etc - they should be included in the backup, but it's best to have a copy of them outside of the backup just in case. You can then force a backup of the iPad via File > Devices > Back Up.
Then start the update by selecting the iPad on the left-hand sidebar, and on the Summary tab on the right-hand side clicking the Check For Updates button
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.
When iOS 7 is released, you can update via wifi.