Go into iTunes and open up the preferences window, and click on importing (iTunes->Preferences->Importing).
Find the song you want to change from MP3 form to AIFF format and control click on it (you can also right click on it). Select "Convert selected song to AIFF". Click that and you're on your way!
You can do it in iTunes, but why? You're not going to get back what was lost in the original encoding--you'll just have a big lossy file instead of a small one.