Here it is from the get-go:
1. Start Audacity
2. Load an mp3 file
3. Using your mouse, highlight or select the entire song
4. click "EFFECT" on the top menu bar
note: you wont be able to use any effects unless a sound file is loaded.
5. Select "change tempo"
6. Select the method to change tempo
7a. click "preview" to listen to a short sample of the outcome
7b. click "OK" the change the tempo of the selected region of the song
8. Save the the new file under a different name to preserve the original