To turn on your bike's headlight, first make sure both the battery as well as the LCD display are powered on.
Once they are, simultaneously push the UP ARROW and the MODE button for roughly 1-2 seconds. This will activate both the front headlight as well as the backlight on the LCD display. Time for some night riding!
Riding a bicycle at night is much more dangerous than riding during the day. Check reflectors/lighting equipment and their mounting hardware regularly to make sure they are clean, straight, functional and secure. If you choose to ride in poor visibility conditions, first check to ensure you comply with all local laws regarding night riding.