To enter programming mode all you have to do is hold the up and down button for 5 seconds when the display is on. It will take you into programming mode, from there press the mode button to navigate different settings and up and down to make adjustments for wheel size, max speed limit, km/h or mph and backlight strength. Hold mode for 3 seconds to save the settings and exit programming mode.
If you wish to know more about the Display functions, check this video out.