The best way to do this is to insert a tempo change at the beginning of the song. In SONG mode, follow these steps:

1. Press the RECORD button
2. Set the RecTrack to TEMPO
3. Press STOP
4. Press EDIT
5. Press F5 INSERT
6. use the arrow keys to move the cursor to the tempo. Change the tempo as desired, and press F6 SET.
7. Exit from the Edit menu and STORE your song.