How do I adjust this?

[carlosfandangus]

+1 my cannondale is exactly the same, loosen off the rail clamp bolt, screw the adjuster in or out and then tighten up the clamp bolt, it takes a few goes to get it right.

You need to loosen the rear bolt to allow some movement on the saddle, make the adjustment required on the adjuster in your picture, then tighten the rear bolt again.

You may have to do this a couple of times as tightening the rear bolt up again will slightly change the angle you’ve dialled in on the front one.