On the Starboard side of the stairway, just aft of the door, inside a hatch is a handle, push handle to closed position. Providing right hydraulics pressurised up comes the stairway. when in the fully up position it is possible to move the handle into the closed position where it latches into a detent.
Lowering is the reverse, although usually done by ground staff for safety using the external handle which is linked by a cable (inevitable on a Douglas) to the inside handle. Stairs will free fall without hydraulic pressure but may need "persuasion"