Replacing a rotted deck post can be a big job, as it’s often tied in to the deck at a number of locations, forcing you to dismantle half the deck to get at the post.
So try and replace the rotted section only.
Of course, you can’t just cut it out and toenail in a section of 4×4. To do this properly you need to create a sturdy lap joint.
Rent a jack, set it on a firm footing, and adjust for the height of the deck.
If there’s a metal saddle or bracket holding the post to the concrete post or pier, make your cross cuts on the “open” side of the saddle. This will make it easier to remove the damaged section.
Using a circular saw, make the first crosscut a couple inches above the rotted section of the post. Extend the cut to exactly halfway through the post with a handsaw. From this point, rip up the center of the post — on either side — with your circular saw. Then make your final crosscut.
Remove any bolts holding the damaged section to the saddle, and knock out the damaged section. Use this piece as a template for your replacement section, allowing for the kerf from the saw blades. With a chisel, clean out any material left by the saws, and insert the new section in place. Drill and countersink holes for three bolts, and attach the bolts.
Replace the saddle fasteners.