Use an online audio converter
Upload the M4A file
Select MP3 as the output format
Start the conversion
Download the MP3 file
Use VLC Media Player
Open VLC
Go to Media
Select Convert/Save
Add the M4A file
Choose Convert
Select MP3 as the profile
Start the conversion
Save the MP3 file
Use Audacity
Open Audacity
Import the M4A file
Go to File
Select Export
Choose Export as MP3
Save the file
Use iTunes or Apple Music on a computer
Import the M4A file
Go to File
Select Convert
Choose Create MP3 Version
Locate the new MP3 file
Use a dedicated audio converter app
Open the app
Add the M4A file
Choose MP3 as the output format
Convert the file
Save the MP3 file