How to update my iOS 4.2.1 iPod to iOS 5 or iOS 6?

Okay so I'm using this old iPod touch and it's on version iOS 4.2.1 and I want it updated to iOS 5 or iOS 6. I have the latest version of iTunes too and I click on my iPod, I click "Check for Update" it says "This version of iPod software (4.2.1) is the current version". And that's all it says. Also when I go to Settings on my iPod there's no option to update it. How do I update it? HELP!