It can either be taken apart and cleaned or you can replace the switch with one from maplin...
De-solder it and solder the new one in it's place. Be sure to cover the contacts on the battery side of the board or you will short it and drain a new 9v overnight...
To clean it, you need to cut the tabs holding the button together, take the parts of the switch out and clean the pcb contacts and the copper disc that makes the contact when you press the button.
To re-assemble, just put it back in the opposite order. You will need to use a decent glue to hold the switch together, but sparingly.