Cut the old threshold in half with a circular saw being carefull not to go to
deep.
Pry out the peices. Remove the staples from the bottom of the jambs thhhhat
held the threshold in place. If possible remove the interior trim at the bottom
and remove the lowest set of shims. This will allow the jamb to move outwards
to fit the new threshold in. If removing the trim is a pain for you, you may be
able to slide it in from the outside and shim it up tight to the jamb and nail
it thru the threshold into the floor below. Either way works fine.