Depending on the model one of the 2 following procedures should apply.
1) Attain access to the rear of the light cluster by pulling aside interior trim.
2) Remove light units backing, this is held in place with plastic clips that need to be squeezed together.
3) remove and replace bulb.
4) Replace backing ensuring all bulbs go back into their respective holes. And replace trim.
2) remove bolts securing light unit to the vehicle. Located inside the boot.
3) remove light unit but pulling (firmly but gently) the unit straight backwards from the rear of the car. It may be easier if you disconnect the wiring connector.
4) remove backing plate on rear of the light unit.
5)remove and replace faulty bulb
6) replace bulb unit on the car
7) replace retaining bolts on the rear of the light cluster inside the boot.
8) replace trim.