Oops - sorry, my bad - assumed you meant plug-in lamp (doh!) - knew I shouldn't have replied whilst being off with flu!
You're on about a lighting circuit (and it 'tripped' the breaker) as opposed to me reading 'tripped on a light'! - Sorry.
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Breaker removal (assuming you're using a UK spec board here),
TURN OFF THE MAIN SWITCH (to kill the board) and any other incoming supply main switch if you have one between the supplier and your board,
USE INSULATED SCREWDRIVERS JUST IN CASE - YOU'RE POSSIBLY PLAYING WITH 60+Amps HERE!
Remove the cover
With a LARGE crosshead screwdriver undo the breaker's OUTGOING terminal (it will have the lighting 1mm tail going into it - basically the top screw with the cable going into it)
Remove the tail and carefully move it out of the way, but not putting it remotely near anything else (including yourself)
Remove the Live BusBar cover (plastic cover at bottom of board)
Undo the breaker's INCOMING terminal (it will have the copper busbar going into it, which is the live connection direct from the main switch - WHICH YOU SHOULD HAVE SWITCHED OFF)
You should see a small plastic loop at the back of the breaker that will just about fit a small 'terminal type' screwdriver into it - place screwdriver into that and give it a nudge up or down (dependant on placement) away from the breaker (so, if the loop is on the bottom - push down, on the top, lift upwards)
If you've a decent board, the breaker should come free (towards you and up) - if it's a cheapy then you'll have to remove the busbar from the lower terminals of all the breakers (which is nasty!).
Stick the new breaker in (guessing B6 rated), making sure it has the same locking mechanism on the rear
Reconnect the busbar (nice and tight)
Replace the busbar cover
Reconnect the lighting tail(s) - (again, nice and tight)
Replace the cover
Make sure the breaker is off
Main Switch ON
Breaker test ?
Still trips - get a sparky in, it's a little more serious.
Kevo (having re-read your original question properly!), April 2008