How do I purchase iOS?

Your current iOS version is shown in Settings > General > About > Version, and to update to a higher is free, you don't need to buy anything nor download an app.

If your iPad is already on iOS 5+ then you should have a Settings > General > Software Update option on it for direct update on the iPad, if you have iOS 4 then you will need to do the update via your computer's iTunes (you can also update via your computer if you are already on iOS 5+).

To update via your computer, connect the iPad to your computer's iTunes and copy any purchases off the iPad to your computer via File > Transfer Purchases. 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, 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

An iPad 2 will update to iOS 6.0.1 (which is the only version that is 'signed' by Apple for it), so you will lose the built-in YouTubeapp and the Maps app will change - though google do have apps in the store.