Take the 2 screws off the cover houseing (should be one on top and one on the bottom) and then squeeze it some so it pops up off the base. Be careful to not squeese too hard or you could crack the plastic housing.
Look at the bottom section of the shifter (you may need to push the thottle full forward) and you should see a small standard set screw setting there, loosen or tighten that screw till you set it how you like it.
Now most of the time you can't get a good feel for it unless you are running the boat, so take a screwdriver with you on your next trip and test it after you bounce over a few waves
Hope this helps,
Dave