Q: I have an iPod 2 with iOS 2.2.1 How can I update to iOS 4.2.1? I get an error in iTunes when I try to do so.

To update

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

iOS: How to update your iPhone, iPad, or iPod touch

A 2G to 4.2.1. Requires iTunes version 10.X.

If your iPod does not have volume buttons on the upper left edge it is a 1G. Those can only go to 3.1.3 by

Purchasing iOS 3.1 Software Update for iPod touch (1st generation)       

https://buy.itunes.apple.com/WebObjects/MZFinance.woa/wa/touchLegacyLandingPage