Buy a new switch ( B+Q, etc). Switch off the elecric. Undo the cover on the old switch. Disconnect the wiring and unscrew the old fitting from the ceiling. Screw the new fitting to the ceiling and reconnect the wiring. Fit the cover to the new switch and turn on the elecric. Only attempt this if you feel competent, otherwise get an electrician in.
Tony T, December 2008