Before the bulb can be replaced, the plastic cover over the headlamp must be removed.
Never touch the glass part of the bulbs with your fingers. Grease from your fingers is vaporised by the heat, coating the reflector and then causing damage.
Detach the headlamp's rubber cover for the main beam bulb.
Detach the bulb by turning the bulb holder upward and then pulling straight out.
Carefully prize the plastic cover at the connector's lock lug so that the lock lug releases.
Unplug the connector from the bulb.
Replace the bulb.
Fit the bulb in the socket and screw in.
Refit the headlamp's rubber cover.