They are just like a bayonet cap light bulb but with three locking bits. Just grasp the black holders and twist like a bulb, anti clock I think but cant remember for sure, and the lampholder and bulb will pull out. Just have a good luck and you'll see, it is really easy. Not sure but you may have black rubber covers over the holder, if so just ease off will a little bit of pulling.
Take a look at the back of the headlamp and you should see where the wiring connects to the bulb holders. Find the one you want and give the holder a twist to unlock it (I think they only turn one way) and then remove it by pulling back towards the windscreen. It's a little like changing a bayonet fitting light bulb.