Step 1: Enter the App and Start Up Your Desired Game.
Step 2: Once the Game Starts, Press the Back Arrow on the Bottom-left of Your Device.A Toolbar Should Come Up.
However, to further change the speed, tap the button that looks like 3 dots one on top of another. a small menu will appear. tap on the "game speed" button on the new menu. a bar will appear, allowing you to change the speed from 10%(super slow, not recommended) to 300%(super fast!!!)
I hope this helped you. Have fun playing your N64 game at High or Low speed!