In iTunes select you iPod, then the Music folder and next the file(s) you want to convert. Right click on the selection, then "Convert ID3 tags" (please note that I'm using the italian iTunes to the translation may be different). Select "ID3 tag version" checkbox and v2.2 (or v2.3) from the list, then simply press ok. If the option is grayed (disabled) your files are not in mp3 or aac format....
Another useful utility for manage tags is mp3tag (
http://www.mp3tag.de/) but works only on windows: in this case you must convert the tags before copying the file on the iPod.