It looks like you had the Plumber from Hell that I hired and fired once.
You should be able to go to the manufacturerís web site and download the installation manual. Sometimes they have repair and parts manuals also. Or call the 800 number.
The button connects to a clear plastic air hose under the tub. You should have an access panel to get to it. The other end of the air hose goes to a switch box mounted underneath somewhere, off the floor. When you mash the button you compress air, which in turn cycles an electric switch, eliminating any possibility of getting your wet naked body in contact with Old Sparky.
The button is housed in an escutcheon plate that can be pried off from the top or pushed from below, and maybe after releasing a some kind of a plastic clip. After you get it off, remove the air hose, then repair or replace the button.