In case it's not clear... if you haven't put in the little locking tab, which most people don't do, you just turn the head of the thing while it's in the tank and it will unlock. You can then set the height by sliding it up and down, then turn it back until you feel it click into place. It moves up very slightly when you click it into place. If you have it positioned in the tank such that you can't turn the head far enough to unlock, you still don't need to remove it. You can turn off and unscrew the water supply, put a bucket under the hole, flush, and then loosen the locking nut just a bit. Then hold the valve in place and turn the stem from under the toilet until it unlocks. Then adjust it, lock it back in position by turning the stem, tighten the lock nut, reinstall the water supply and turn it on, and voila.