Your Presence has three buttons on the back.
The circular button at the top controls the backlight of the display. Note that it does not power the display on and off, so you must unplug it to turn it off completely.
The long button located below the circular button lets you manually navigate between the Rays that are currently displaying on your screen. As you move back and forth through the Raylist, the app will also update to stay in sync with your display.