Along the circumference of the flywheel, there's some adjusting bolts with lock nuts on them that push on the secondary clutch plate.. the clearance is supposed to be 0.090" plus/minus 0.002"... the most important thing is they're all even. I personally like the secondary clutch to come out sooner rather than later, so if I'm not happy with .090" I might tighten it up to 0.080" so it's easier to disengage the PTO
The manual shows this procedure being done on a flywheel on the bench, but I'm quite sure there's a window in the bell housing you can access it from, there's 3 fingers, so you'll have to rotate the engine, access to these bolts is from the sides of the flywheel.