I'm in Windows 2000 so it might be slightly different, depending on the operating system.

Open any folder.
In the folder's menu go to Tools > Folder Options.
The Folder Options window should have several tabs near the top of the window, select the "View" tab.
Uncheck the box for "Hide file extensions for known types".
Press "OK".

Now you should be able to simply rename it. Change the name from [Insert Filename Here].mp3 to [Insert Filename Here].txt. It shouldn't change any of the data within the file.

EDIT: If your not use to being able to view and edit the file extensions, you should probably re-check that box when your finished. While you can simply edit the filename a second time if you screw up, it would be bad if you renamed something important and "forgot" to fix it and/or change it back to its original state.