iOS is the operating system that runs the iPod. If you really have a 3rd-generation iPod touch, connect it to iTunes on your computer, select the iPod in the sidebar, and click the Check for Updates button. See the section titled "Update your device using iTunes" in this article:
Confirm that you really do have a 3rd-generation iPod, though.
A 2nd-gen iPod touch can only to go iOS 4.2.1.