This relates to the most popular hatchback model.

1. Open the boot.
2. On the edge of the boot opening, next to the rear lamp cluster, there is a round plastic cap.
3. Carefully prise out this cap, and this exposes the bolt holding the lamp cluster to the car.
4. Unscrew the bolt, then carefully lift the lamp unit free of the car body.
5. Remove the brake/tail lamp bulb holder from the back of the lamp unit, replace the bulb, put the bulb holder back in place in the lamp unit.
6. Replace the lamp unit in position, re-fit the bolt, then refit the round plastic cover.

Heres a video tutorial off the process. Its in French, but still useful!