The knurled ring in front will change the shutter. Because the IIIc "couples" aperture and shutter speed, the aperture will change as well. To change the aperture without altering shutter speed (and I suspect this is the basis for your question), you lift on the small silver tab at the bottom of the lens and move it to change just the aperture.
Ohhh ... the aperature markings are below the lens... that small silver tab arrows toward the aperature at which it's set, right? Thank you for the info! ... and thanks to Ian for sending the manual.