Are you talking about the threshold? this is the bottom piece of wood that is on the floor area of a exterior door.
here is what I would do: carefully pry out the old threshold. This is the hardest part of this job and one that may tax your patiece. Try to get it out in one piece if possible.
Once the old threshold is out, Trace this old piece onto a new stock of presure treated wood. Cut it out and now prep it for the door.
I would prime the threshold and then paint it. Then I would install a nice metal threshold cap to that. These have nice rubber insulation pieces that will keep the water and outside air out.
Slide the new threshold into place and fasten it down. do your caulking and then install the new metal cap.
You might have to shave the bottom of the door to accomodate the new threshold. if you do be sure to paint or seal the new door cut. Only if it is wood. This will keep it from changing shape due to weather.