Open the door completely.
Remove the screws holding the threshold in place with a drill.
Remove the old threshold and vacuum or sweep out any collected dirt and debris.
Measure the old threshold or, if it is bent or severely damaged, measure the threshold space.
Cut a new aluminum threshold to size with a hacksaw. File the edges smooth with a metal file.
Drill two or three holes evenly spaced in the center of the new threshold. Set into place and drill out holes in the wood door frame corresponding to the threshold holes.
Remove the new threshold and caulk the perimeter of the threshold space.
Set the new threshold in place once again and drill into place with wood screws.