There are two screws in the bottom of the case to make this adjustment. To make the adjustment, remove the jack rail so you can watch the jacks move up and down as you turn the screws. Try to position your self to observe the jacks as you turn the screw with a screwdriver. If jacks are too high, turn screw counterclockwise until the dampers make contact with strings. Play a few notes at the end you are adjusting.
Some players prefer absolute damping, others allow a little resonance.
Adjust the second screw and check dampers throughout the keyboard range. Make any further adjustment. Be sure not to lower the keyboard too much. Check by moving handstops on and off. The dampers should not push strings sideways.