What year is your ZX-6R? Some have a nut and some have a shim. Either way you have to remove the nut on top of the shock mount that is under the seat. Then you will have to get a couple of strong friend to lift the rear of the bike so the swing arm can drop down far enough to get the shim/nut off or take the bottom of the shock loose to do the same thing. Removing the shim will lower the ride height about twice the thickness of the shim. To correct the handling you should raise the forks above the top of the tripple clamp about twice the thickness of that shim. You will lose a little ground clearance but for most of us mortals it will not make a big difference.