Open the Audacity program. Find your mp3 and drag and drop it into Audacity. You can drop the mp3 on any part of the Audacity window and the program will automatically import the file.
Navigate to the title of the track as it appears next to the wave form. Select the downward arrow and a drop-down menu will appear. Select "Split Stereo Track." This will split the tracks into individual audio tracks that you can edit.
Select the second track that says "Right" by simply clicking once on the track. Click "Effect" from the menu bar at the top of the screen. In the drop down menu that appears, click "Invert."
Set both tracks to "Mono." Click on the down arrow next to each track title. In the drop down menu that appears, select "Mono."