Install an audio-editing program. If you have Mac OS X, you might already have a copy of GarageBand in your "Applications" folder --- it comes standard with the Leopard and Snow Leopard operating systems. If you want a free audio-editing option, try Audacity, Traverso or Qtractor. (See Resources)
Open your audio software and import the song that you want to edit. In some programs, you can drag the song file directly onto the project window, and it appears on your screen as a horizontal colored region with wavy lines in the middle. In other programs, you need to select an "Import" option, usually found under the "File" menu at the top of your screen.
Click your mouse inside the "Tempo" box. The location of the box varies among programs but usually appears along the top or bottom of the main project window. It might read "Tempo" or "BPM." Most programs use the number "120" as the default tempo.
Enter a new tempo, lower than the one currently appearing in the "Tempo" box. For example, if the box currently reads "120," you might try entering "105" as a replacement. In some programs, you see a horizontal bar beneath the tempo box with a dial that you can drag to the left or right to reduce or increase the tempo.
Click the "Play" button to hear how the song sounds. If you reduce the tempo too much, the quality of the recording suffers. If you notice that the recording sounds distorted, try a smaller tempo reduction.
Export your project as an audio file. When you have the tempo exactly as you want it, resave the project as a self-contained audio file, like an MP3 or WAV file. You can usually find the "Export" option under the "File" menu. Choose a format in the pop-up window that appears, and click "Save" or "Export" in the lower right corner.