How do I update ios on Ipad?

This is because you have a 1st Generation iPad and this device’s hardware is not capable of running any iOS higher than 5.1.1 so it has the highest version available which is why it is telling you it’s got the most current version.

A first gen iPad, which the model number in your tagline is, only supports up to iOS 5.1.1, it can't be updated any higher