Apple doesn't release iPod OS updates as often as it does for the iPhone. That makes sense; fewer iPods are sold these days and new models come out less frequently, so there are fewer changes to make.
You can update iOS devices like the iPhone or iPad wirelessly over the Internet. Unfortunately, iPods don't work that way. The iPod operating system can only be updated using a computer.
This article explains how to update the operating system on the following iPods: