Use a file converter app or website that supports MPEG4 audio to MP3
Upload or select the MPEG4 audio file
Choose MP3 as the output format
Start the conversion
Download the converted MP3 file
Use VLC Media Player to convert the file if you prefer desktop software
Open VLC
Go to Media
Select Convert/Save
Add the MPEG4 audio file
Click Convert/Save
Choose MP3 as the profile
Select a destination filename ending in .mp3
Click Start
Use FFmpeg if you want a command-line method
Run ffmpeg -i input.m4a output.mp3
Replace input.m4a with your MPEG4 audio file name
Replace output.mp3 with your desired MP3 file name
Check that the converted file plays correctly
Make sure the original file is not protected by DRM before converting