So M4A to mp3 … how to start …
Start form getting ffmpeg into you’re system.

https://www.gyan.dev/ffmpeg/builds/

Install ffmpeg

winget install ffmpeg

Now restart console. On top of explorer window type "cmd" this should open comand line in you're folder which you are now.

ffmpeg -i input.m4a -c:v copy -c:a libmp3lame -q:a 4 output.mp3

For more nice things take a minute to look at: