it is a long and badly designed process mate.
Get on your back seats, and look for circular cap, pop it off with a flat end screw driver and you'll see a bolt, undo it.
Open your boot, and there will be the same sort of arrangement, just lower down, get the cap off, then undo the bolt.
Lift the light off the car, and unplug the connector. Now, you have to undo the screws in the back, to separate the light tray from light itself.
Eventually, after a lot of swearing, sweating and maybe even tears, you can change your rear bulbs.
Then all you have to do is go through it all again to put it back!