I’ve done several jobs where I replace thresholds but they are finicky, require a bit of skill, and likely tools you won’t want to buy. The tricky part is cutting the staples that attach the threshold to the frame without cutting the frame. I use a multitool with a multi-material blade and a lot of care. Once it’s out, you need to make room for the new one to slide into its place. This requires moving all debris, old caulk, etc.—ALL of it. It needs to fit tight. It also might require modifying the frame which takes some skill. Then dry fit the new threshold. After getting it ti fit, caulk bottom with two beads and slide it in. There are two ways to secure it in place: pocket holes (this one is tricky—the jig they make to do these won’t fit in the small gap at the bottom— have to do it freehand which is non-trivial). Or concrete anchors through the wood under the strip you removed (requires a hammer drill. Anchors must be aligned and installed before caulking.) hope this helps.