Before you do anything isolate the electrical supply to the lighting circuit, either remove the fuse or switch off the circuit breaker. You have 3 choices,
1 drill the heads of the screws off, and replace the back box and switch
2 break the switch and replace the back box and switch
3 Get a competant person to do it for you.
LETS FACE IT IF YOU HAD NOT THOUGHT OF OPTIONS 1 AND 2 SHOULD YOU REALY BE DOING THIS YOURSELF?
ELECTRICITY KILLS YOU ONLY HAVE THE ONE LIFE TAKE CARE OF IT.
pete
July 2008