Watch this video from This Old House to learn how to replace an old threshold.
1 Remove old threshold from doorway using hammer and reciprocating saw.
2 Use reciprocating saw to cut the nails securing toekick; remove toekick.
3 Test-fit new threshold in doorway.
4 Build up toekick and subfloor with pressure-treated lumber; re-install toekick.
5 Apply bead of tri-polymer caulk to joint between finished floor and new threshold.
6 Install new threshold, then tap shims underneath.
7 Close door and check for even gap along threshold.
8 Open door, and apply expanding polyurethane sealant under the threshold.
9 Close the door, then tap shims between door bottom and top of threshold; leave shims in place until the sealant cures.
10 Sand threshold, then finish with an exterior paint or stain and topcoat that contains a ultra-violet protector.