Take the lid off the tank and set it out of the way on a sturdy surface.
Locate the float in the tank. Follow the mechanism to which the float is attached to the other end. Here you will locate a screw that adjusts the height of the float.
Turn the screw counterclockwise in order to lower the float. A lowered float will shut off the flapper earlier, letting less water into the bowl. Try adjusting the screw in small increments until the water flushes and fills to the level you want. Put the lid back on when you are done.