Larry asked about changing an audio file's extension to .wav.
Changing the extension is easy, but it will probably render your file unusable, because changing the extension doesn't actually change the format. It just gives Windows inaccurate information about what type of file you're trying to launch. The most likely effect would be to render the file unusable.
If you really wanted to convert an audio file from one format (say, .mp3) to another (say, .wav), you'd have to convert the audio. You can do this in an audio-editing program like Audacity. Just load the file, then export it to another format.
But I'm not sure why you would want to do that. A .wav file is much bigger than an .mp3 of the same recording. While .wavs generally sound better than .mp3s, that won't be the case after a transfer. A .wav created from an .mp3 can't sound any better than the .mp3 it was created from; the lost data is already lost.
Converting from .mp3 to another lossy compressed format, like .wma, is even worse. The process will likely lose you even more data.
Getting back to your original question, there are legitimate reasons to change a file's extension. Perhaps someone else changed the extension incorrectly. Or they did so intentionally for security reasons. So here's how to...let's say, correct an extension:
First, you must make your extensions visible. In Windows Explorer, select Tools or Organize (depending on your version of Windows), then Folder options or Folder or search options. Click the View tab. Uncheck Hide extensions for known file types, then click OK.
Now you'll see the extension as part of the file name. When you select a file and press F2 to edit the file name, you'll be able to edit the extension, as well.
While we're on the subject, try this trick: Find a .docx or .xlsx file and change the extension to .zip. Now open the file. Those Office documents really are just .zip files. But change the extension back so you can load it into Word or Excel.
Contributing Editor Lincoln Spector writes about technology and cinema. Email your tech questions to him at email@example.com, or post them to a community of helpful folks on the PCW Answer Line forum. Follow Lincoln on Twitter.