How to turn pinhole wheel in Zero image 75B camera

Can someone please tell me the proper way to turn the aperture/ zone plate wheel behind the lensboard of a Zero Image 75B Deluxe camera.
I can hook the fingernail of my index finger under the wheel and turn it, but I am afraid of damaging it. Must be an easier and safer way of turning it.

Many thanks,
Sam Hotton

i just put my finger on the wheel and turn it gently. i kind of use the "indents" of the cut outs to gain the neccessay friction to turn it. press gently.

eddie