How do I replace a rotten threshold underneath my back door?

I cut the center out with a sawsall, separating the 2 pieces and clearing the debris from the bottom of the door frame,including nails that held the old threshold in place.

A new threshold can then be measured and fit to the opening.

Before installation of a new piece,caulk the base of the opening with high grade silicone,it'll keep bugs and moisture from doing the same thing to the new piece.

On top of the piece,paint or seal so that it doesn't rot again.(caulking where the sides meet the new threshold)

You might also consider using a treated lumber threshold instead of standard wood, depending on the area its located.

Hope this helps,

Ken