Open iTunes.
Click "iTunes" and choose "Preferences" to open the iTunes preferences menu.
Click the "Import Settings" button.
Choose "MP3 Encoder" from the "Import Using" menu and then click "OK." This will determine what audio format you prefer to import and convert audio files to with iTunes.
Click "OK" to exit the iTunes preferences menu.
Find the purchased song you would like to convert to MP3 in your iTunes library and click on it to highlight it.
Click "Advanced" and choose the "Create MP3 Version" option. Once the conversion is complete, an MP3 version of the purchased iTunes song will appear directly below the original. You have now successfully converted a purchased song in iTunes to MP3.