how to upgrade ios 5.1 to 6

If you have a first generation iPad you can't. It maxes out at 5.1

If you have an iPad 2 then you can either do the update over the air or via iTunes.

If you have an iPad 1st generation, you are already at the maximum iOS version that the iPad is capable of running. You cannot update the iOS beyond iOS 5.1.1

If you have an iPad 2nd generation, your next update will be to iOS 7.x.x. You cannot update to any iOS version in between 5.1.1 and 7.x.x. You have to go directly to iOS 7.x.x