You can change the format from m4a to mp3 if you want by re-importing the tracks. It is recommended that you re-import from the original source, as converting a lossy format to another lossy format is bound to cause severe deterioration in sound quality. To import as mp3, you have to go into iTunes preferences and change the relevant settings under 'Importing'.
AAC is generally regarded as a superior format to mp3, however mp3 is a more universally recognised.