My iPad does not have the software update icon but I want to do an update.

The Settings > General > Software Update option only appears when you have iOS 5+ installed, as you have iOS 4 then you will need to do the update via your computer's iTunes.

Connect the iPad to your computer's iTunes and copy any purchases off the iPad to your computer via File > Transfer Purchases (File > Devices > Transfer Purchases on iTunes 11). You may also want to copy photosand any important documents off the iPad as well e.g. via the file sharingsection 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 havea copy of them outside of the backup just in case. You can then force a backup of the iPad by right-clicking the iPad 'Device' on the left-hand side of iTunes and selecting 'Backup'.

Then start the update by selecting the iPad on the left-hand side of iTunes (on iTunes 11 you can enable the left-hand sidebar via option-command-S on a Mac, control-S on a PC), and on the Summary tab on the right-hand side clicking the Check For Updates button

Updating to iOS 5+ : http://support.apple.com/kb/HT4972

Your iPad should be updated to iOS 6.1

Connect to computer and update with iTunes