Converting .mp3 audio to .aac format.
Converting MP3 Audio Files directly to Advanced Audio Coding Files (.mp3 to .aac conversion) is possible and can be done using one of many available audio editing programs or conversion tools.
Try, for instance, AnyToAudio Converter or Helium Music Manager, which should be capable to easily convert .mp3 to .aac.
Alternatively, you may try some online conversion solutions.