turn off the mcb breaker for the light circuit in the consumer unit...
take hold of the bulb by either end and carefully rotate it through 90 degrees, it should then slot out..
partially undo the 2 screws youll see and remove the cover its usually on a keyhole slot
inside youll find a strip connector with what should be brown, blue and yellow/green... undo the feed cable (the one from the ceiling) if theyre not this colour theyll be black, red and green (old style wiring, makes no odds)
remove the fixing screws from teh back of the unit and slide the unit over the feed cable...
either leave the mcb of till your done and replace in reverse or put another strip connector over it and tape up with insulation tape then switch back on..