Bulbs with a bayonet fastener must be changed as follows:
Removing the bulb
Lightly press the defective bulb into the bulb holder, then turn it anti-clockwise and remove it.
Do not touch the glass part of the new bulb with your bare hands, use a cloth or paper towel instead.
Fitting the bulb
Fit the new bulb, pressing it into the bulb holder, and turn it clockwise as far as it will go.
If necessary, use a cloth to remove any fingerprints from the glass part of the bulb.