Power on the screen then press and hold the bottom left button (next to the power button) for a few seconds.
When it goes into the settings mode, press that button again once.
You should then see a screen with flashing km/h.
Press the Up arrow button and that should change to a flashing MPH.
Press the bottom left button to confirm the change.
Then you can power off/on the screen to reset and you should be in MPH.