There are two settings to turn on for the tempo to change by 5% when tapping the +/- buttons, one is enabling the buttons, and the other is turning on snap speed slider.
Open the App Settings
Open the App Main Menu
Select Settings
Check the box to turn on Plus / Minus buttons
Tap on Pitch and tempo
Check Snap speed slider
Now when you tap the + button in the editor, the music speed increases by 5% increments