If you have an iPhone or iPad, the PianoTrax app will change key and tempo for you. The Android app does not have this feature yet.
Whether you have purchased a song from the app or the website, it will automatically be in the PianoTrax app. When you log in, click the "My Trax" folder and choose your song. Click the "edit" button. The song will automatically download the track to your mobile device and a wheel will appear with options to change key and tempo. The app will remember your modifications.
The app will not export a downloadable Mp3. If you need an Mp3 with these modifications, contact [email protected]
and request one. Tell us the song title, key and length and the new key and tempo you need. There is usually no charge for me to transpose or change tempo if I have the original MIDI file I used to record the song.