There really isn't a way to "convert" an M3U file into an MP3 as the M3U is just a link.
If you still have access to the M3U file, you can still use it to find and save the MP3 it links to. To do so, right-click on the link that opens the M3U and tell the computer to "Save Target As" onto the desktop.
When the M3U is finished saving, open up NotePad or WordPad and drag the file M3U file onto it (this will open it). Inside of the M3U file, you will find the link to the MP3 that it was meant to stream.
Just highlight and copy this URL and then paste it into your browser's address bar. That should take you to the MP3 that you were looking for.