You may adjust the playback tempo of your sheet music using the "Tempo" button from within playback mode. You can change the tempo both during playback and while stopped.
To adjust the tempo, you may tap one of the pre-selected speeds (relative to the piece's default tempo), use the slider, or type a number into the BPM field. You can return to the default tempo at any time by using the "1x" button.
When you exit playback mode for your piece, the tempo you chose will be retained for the next time you open it (even if you sign out, close the app, or remove the song from your download list).