When viewing or playing a score in the mobile app, the cog (⚙) icon on the top-right corner of your screen takes you to the score's settings.
The Tempo slider that can be found at the "Metronome" tab allows you to adjust the playback speed as a percentage of the original tempo from 10% to 200%.
On the same tab you can adjust the Metronome volume and turn it on/off.
Under the "General" tab, you can transpose the score into a different key by tapping the < -/ +> buttons to cycle through key signatures. The "Default" button returns the score to whatever key it was originally.
To go back to the score, just slide this tab down.
Keywords: tempo, transpose, metronome, countdown, transposition.