@pete - It's a really easy job! It's specific to the type of car, but there'll defo be a youtube on how to do it, as @GARYXKNIFEDX says. The only thing to consider is that you need to be careful how you manoeuvre the light itself during the process...too much movement can throw the focus out, which can make you have a minor fail at the next NCT, and be irritating to drive past at night time.
This might help (haven't looked at it mind you):
Happened to us recently - we've both been replacing bulbs ourselves for years, in general the pre-NCT guys have fixed the focus for us without mentioning it. We got snared at the last NCT - but the guy actually fixed it for us during the test and let us through. It was brilliant.
So anyway i took the faulty one out and bought a matching pair in halfords and decided to install the faulty one there and then myself. in the blanch centre car park. in the dark. It did not go very well.
30 minutes later i finally had it in... and then the other one stopped working.
Bought a van, disposing of the car relatively soon. First impressions, very chill on motorway, doesn't feel fast which kinda explains the whole van man thing. Fuel tank is rediculously big compared t a car for humans. haphazard camper van project is imminent.