Open the trunk
Pull back the interior trim in the corner and you should see the housing for the tailights.
Remove the fasteners by using an 8mm socket, and remove the housing from the body of the car
Once you've done this, remove the bulb you are replacing by firmly pressing the bulb socket while turning it counter-clockwise about a 1/4 turn and remove the socket.
Remove the bulb from the socket and replace it with the new bulb.