Converting iTunes File Formats (I've never used iTunes, so am assuming the following will work):

1. Open iTunes
2. Click on iTunes – Preferences
3. Click on the Importing button at the top of the window
4. From the Import Using pop-up menu, click on the encoding format (AAC, AIFF, Apple Lossless, MP3, WAV) to which you want to convert your music files
5. Click on OK to save the settings
6. Select one or more songs in your iTunes library to convert (Command-Click to select specific individual files or Shift-Click to select files in a row)
8. Click on Advanced – Convert Selection to... (the format you selected in preferences above is displayed)
9. A copy of the file (or files) will be created in the new format