Check for the location of cracks between an exterior door threshold and an exterior door on a sunny day. Get on your hands and knees in front of the door on the inside and look between the threshold and door to see where the sunlight comes through. A little light at the corners is acceptable.
Open the door and adjust the threshold by turning the screw closest to the spot where light came through. Turn the screw counterclockwise with a Phillips screwdriver to raise the door threshold.
Lay the edge of a framing square over the door threshold to see if it's straight and level. Adjust any other screws to the necessary height to make the top of the threshold level. Turn screws clockwise to lower a section of the threshold if necessary.
Close the exterior door and check for light shining through from underneath after making the adjustment. Make further adjustments if necessary to close the crack under the entry door.