How can I just update the OS of my iPad 2 (4.3.3 now) to iOS 5 but not iOS 6?

You can only upgrade to the most recent version, in this case 6.0.1.

You need to connect it to your computer and iTunes and click on the update button that shows up on the details of the iPad screen. Up until iOS 5 this was the only way to do the update. After iOS 5 then you can do it wirelessly on the iPad without iTunes.