Hold UP, and select Settings > System > Backlight.

Mode

Sets the backlight to turn on manually or automatically. The Auto option allows you to separately set the backlight for keys, alerts, and gestures.

NOTE: When you set Gesture to On, you trigger the backlight by raising and turning your arm to look at your wrist.

Timeout

Sets the length of time before the backlight turns off.

Brightness

Sets the brightness level of the backlight.