how to convert aac files to mp3 files

Open iTunes, and in Itunes Preferences : General, click Import Settings… Change the import using selector from its current setting to MP3 Encode, and leave High Quality, and Use error correction alone. Click OK.

Now, select the AAC song track, and then from the iTunes File menu, select Convert > Create MP3 version. This will make an .MP3 version of your selected artist track, with the same name, in the same Artist folder.

For Catalina and Big Sur, see Convert music file formats in Music on Mac.