Unless your iPad is already on iOS 5 or higher, you won't find any option to upgrade on the iPad itself. You have to connect you iPad to your computer and use iTunes to do the update. Follow method 2 in the Admiral's link to update your iOS.
What people are trying to say here is that you'll need to plug your iPad into your computer’s USB port to let iTunes update it. If you have an iPad1, you won't be able to update to iOS6. Dunno if it would offer you iOS5 instead. Owners of iPad2 or iPad3 cannot get iOS5, and can only update to iOS6.
Did I leave anything out?
milliHelen: amount of beauty required to launch one ship.