rotting threshold under exterior door

Yes you can and it is not a terrible job to do. You just have to be handy :-) These are the instructions from here

1. Open the door and secure with a doorstop or heavy object such as a brick. Put on the knee pads and safety goggles. Wear protective gloves if the old threshold is broken or splintered.

2 Pry away the damaged wooden door threshold carefully with a crowbar to avoid damaging the door's frame.

3 Measure your replacement threshold, and the base of the doorway. Cut the new threshold to size using the saw.

4 Lay the new threshold in the doorway to make sure that it fits well. Wooden shims may be needed to support uneven surfaces. If necessary, notch the ends of the new threshold so that it snugly fits without any gaps in the doorway. Use your sandpaper or an electric sander (see the image above) to sand the cut edges smooth. Apply caulk to the bottom and at either end of the new threshold and press it into place.

5 Make sure to pre-drill holes in the threshold for the finishing nails. Securely nail the threshold to the sill. Countersink the nail heads and fill in the holes with putty. Sand the threshold again and coat it with a moisture resistant clear sealant or polyurethane.

Here is a very good video that should help you further