Same way you rename any file. Right click, select rename. Rename using the
..mp3 at the end. You will be warned about changing the file type, click
okay. Try to play it. If you can't change it back.
If as suggested above you really mean .mp4, you should use one of the
conversion programs. Remember that .mp4 is a video and .mp3 is audio only.
..mp4 is not natively played in Vista due to codec problems. If you want
video, convert it to .avi or .wmv