Lift the mattress out of the crib and set it aside. Remove the four hex screws securing the four corner's of the spring frame to the crib's corners. Some spring frames are positioned using swivel arms. Remove the four hex screws securing the arms in the height position using the hex wrench.
Lower the crib's spring frame to the desired height. Line up the holes in the spring frame's corners with the height holes on the crib's side. If your crib has swivel arms, line up the pivoting arm's screw hole with the opposite arm's height setting hole. Line up each screw hole with the same height hole so the frame sits level.
Insert hex screws through each of the four holes. Tighten the screws in place. Place the mattress back in the crib.