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
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Select Settings
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Check the box to turn on Plus / Minus buttons
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Tap on Pitch and tempo
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Check Snap speed slider
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Now when you tap the + button in the editor, the music speed increases by 5% increments