Clear grass or stone away from the bottom of the deck post. Prod along the bottom of the post with a screwdriver or an awl. If the wood is spongy or pieces easily peel away, you'll need to replace the post. Start by nailing 2x4s or 2x6s together to use as temporary braces. Place scrap wood on the ground for a pad within 3 ft. of the post being replaced, then set a hydraulic jack over it. Cut the brace to size, set one end on the jack and place the other end under the rim joist. Slowly jack up the brace until it's wedged tight. Be careful not to overdo it. You're just bracing the deck, not raising it. If you hear the joist boards creak, then stop. Then place a second brace on the other side of the post (as seen in the photo). (If you don't have jacks, you can rent them.) Or you can set your temporary braces directly on the pads and drive shims between the posts and the rim joist. Mark the post location on the footing, then remove the post by cutting through the fasteners that tie it to the rim joist. Use a metal blade in a reciprocating saw (or knock out the post with a hammer). If there's already a bolt sticking out of the footing, use it to install a new post base. If not, you'll need to add a 3/8- by 4-in. wedge anchor. Do this by placing the post base at the marks where the old post sat, and then mark the center. Remove the post base and drill the center mark with a 3/8-in. masonry bit. Drill down 3 in., then blow the dust out of the hole.