you have to actually open it up, then pop off the knobs and remove every pot and switch nut and the jacks on it and the board pops right out real easy. then flip it over and you see this...
not my pic...
the switch is on the top left. however it is set on the pedal you own is the stock setting. slide it the other way and you get the other pedal since it adds or removes components from the circuit.
they hide it on the top side of the board because we're really not supposed to know about it since you pretty much get 2 pedals for 1 price if you're willing to open it up or even mod it for an external toggle.
some guys have posted that they bent a paper clip in some way to be able to get to the slider switch without even taking more than the back plate off the pedal. i never tried this but i guess it could work if you're too lazy to crack it open lol.
also this won't work for older GT-OD or ZW-44 pedals since the switch is fairly new. my GT-OD doesn't have the switch since i got it when they first came out, but my newer Classic OD does have it.