Download and install Audactity (See Resources.)
Open Audacity, click and drag your MP3 file onto the Audacity interface and wait for it to load.
Click the little arrow next to the channel name and select "Split Stereo Track." This will split the track into two channels.
Select the "Right" track by clicking the gray area underneath the title.
Click "Effects" and select "Invert."
Click the little arrow on each of the channels and select "Mono."