The foot-pedal has a little switch under the toe section. Push the pedal all the way down and then lean on the front of the pedal. It’s designed to be difficult to switch so that you when you are rocking a sweet wahwah you don’t accidentally switch to a pitch shifter! So lean into it.
In some cases, having the pedal on thick padded carpet may prevent you from easily getting this to click over to the next effect. If you have this problem, try having the unit on a sturdy firm surface.