How do you fix a door threshold?

First of all you have to remove the old threshold. If the threshold is underneath the frame of the door, you will need to cut through it on either side of the doorway, as close to the door frame as possible. Be careful not to cut into the floor below. Lift out the threshold if you can because there may still be some nails holding it down. Take out the end pieces of threshold that are under the door frame. Nails may be holding in these pieces and you don't want to split the door frame. Use a hammer and a small chisel or flat head screwdriver to break the threshold ends into small pieces that you can remove easily. Then you can install the new threshold. Insert shims under the threshold to support it and make sure that it slopes downward to direct water away from the door. You may be able to use one long piece or several shims in different thicknesses to ensure your threshold is level. Once you have test fitted everything, remove the threshold and use adhesive to keep the shims where you want them. Replace the threshold, and insert some more shims on the outside of the threshold against the door jambs if necessary. Use a little adhesive here also. Drill holes in the threshold for a few nails. Hammer the nails down, securing the threshold. Coat the threshold with water repelling finish or other sealant and allow to dry allowing foot traffic to return to the area.