How do I upgrade my ipad2 version to 5.1 or higher

You indicate that you already have 5.1..... With 5+ you can update from the iPad via Settings, general, Software update. For lower versions you have to connect the iPad to iTunes on your computer and update via it.

I would re move all paid apps and free apps be for upgrading. All so back up with iTunes.

all so you can do a restore on your iPad 2 wish will give you a nic clean start