That depends on what version iOS you are running.
Plug your iPod into your computer and update through iTunes. After you have the most current iOS you can perform future updates right from the iPod
Settings > General > Software Update
Just to be clear..this option will not be there unless you are running iOS 5 or later.
The Settings>General>Software Update comes with iOS 5 and later.
Connect the iPod to your computer and update via iTunes as far as your iPod model allows
A 4G to 6 Requires iTunes version 10.7 or later. For a Mac, that requires a Mac with OSX 10.6.8 or later