If it's the front Map reading light, tak a small/medium flat bladed screwdriver, and on the rearmost edge,
where the border of the light meets the headlining, place the screwdriver approx 1"-1.5" from the each end
in turn and lever it down quite firmly. It is a very tight fit, if you are unsure, I will try to take some pictures tomorrow.
Once the rearmost edge is released from the two retaining tabs on that edge, the front should have enough movement
in the front tabs to allow you to access the bulb to replace it. I believe the bulb is a
BA9 ?
To replace the cover it just presses back into place and will click when fitted correctly.
if it is the centre dome light, sit in the back and take a small/medium flat bladed screwdriver, place it where
the border meetes the white lens and push the lens forward, you should then see the two retaining tabs,
gently tease the white lens out and replace the
31mm festoon bulb. Job done

To replace remember the edge with one small tab goes towards the front, two tabs to the rear.
If you feel unsure, do not attempt this as I will not be held responable for damage you do to your car.
Let me know how you get on, if you have no luck I will take some pictures to post up how to do it.
(or you could come to Bromley and I'll do it for you

)
regards
Wozza