Insert a thin blade screwdriver between the back of the light unit and the roof. Prise the unit out.
Disconnect the electrical connector.
On the end you can see a tab that holds the lens cover in place.
Insert your thin blade screwdriver and push the lens cover out.
The rear light unit will now be in 2 pieces. The 36mm festoon bulb (12V 7W) is just pulled out.
Replace with the same type or go for a festoon LED replacement (it'll only work 1 way round).
Using a thin blade screwdriver, lever the light unit out of the surround from the back of the unit.
There is a sprung metal clip at the back (lowest in the picture) and clips at the front.
Disconnect the power connector.
There are now 5 clips that need to be released. There is 1 each end that can be unclipped with your fingers.
At the front edge you can use your thin blade screwdriver on the single clip...
...and the 2 clips on the back edge.
With those pushed in, the diffuser will be released and the unit will open. The bulb is visible and can be pulled easily from its holder. It is a 12V, 10W halogen pill bulb. Bulb type 433B, BA9S style. There are no LED replacements.
The internals of the front light show how the light from a single bulb can be directed simply by twisting the central part of the courtesy light. Applying silver foil on the inside of this tube could increase light output.
Refitting is the reverse of removal.