Ringo has two buttons that have the same function.

One is the power button that is located right below the screen.

The other one is a very small button located on the right side of the main board, right next to the white ON sign.

If the phone is turned off, hold either of the two buttons for a few seconds until something appears on the screen.

If the phone is turned on, hold either of the two buttons until a screen pops-up asking you to turn off or restart your phone. Select the desired option and press the A button.