Lift the bonnet and look behind the headlamp in question, there will be a few nuts that are holding the assembly to the front apron, remove them and also the electrical plug from behind the headlamp.
if it is only a bulb you have to replace you will find that where the electrical plug connects to the rear of the headlamp there is a rubber weatherproof boot which you remove, it will expose the rear of the halogen bulb, there will be a little metal spring type arm retaining the bulb,push the end of the spring towards the back of the headlamp and to the side and let it come back away from the headlamp, this will let the bulb come out and you replace it with the new bulb and replace the items in reverse order