In the Redmi note9 mobile phone, taking Format Factory Converter 4.1.28 as an example, you can refer to the following steps to convert m4a to mp3:

1. Turn on the phone, and then click "Open" Format Factory Converter".

2. On the homepage of Format Factory Converter, click "Audio Format Conversion".

3. Click the magnifying glass icon of “Search” in the upper right corner.

4. Click to search for the m4a format file you want to convert, and then click "Next".

5. Select the target format as "MP3", and then click "Start Conversion" below.

6. After the conversion is completed, you can use the above path to view the corresponding file on your phone.