Connect the iPod to your computer and update via iTunes. You will go to 6.1.3
You need iTunes 10.7 or later on the computer.
Your iPod does have cameras, right? The 4G has cameras.
You have either the 2nd gen or the 3rd gen iPod.
The 2nd gen cannot go past 4.2.1. You can't upgrade this model because of the hardware - it is too old.
The 3rd gen can go to 5.1.1 - update with iTunes.
Look on the back of your iPod to find out which one you have.
A1288 is 2nd gen and A1318 is 3rd gen.