A wood fence will eventually rot after many years in the weather. Poor drainage around the posts can create rot. While you can simply reinforce the post when it is rotten, you will still have a rotting post. The best thing to do is to replace the post with pressure treated wood.
Remove the boards from the rotten fence post, using a drill to unscrew the screws. Take the boards down and pile them up, so that you have room to manoeuvre around the fence post.
Loosen the dirt around the rotten fence post. Wrap the chains around the fence post tightly, and attach chains to the front-end loader of a tractor.
Slowly lift the bucket of the tractor up, making sure the chains stay tight, and lift the post out of the ground. Lower the bucket after you have removed the post and remove it from the chains.
Place the new post into the ground. Add quick drying cement to the hole if you want, although it is not necessary if you place the fence post 3 feet deep in the ground.
Fill the hole with soil, tamping around the fence post until the soil is compacted.
Attach the boards onto the new fence post with screws. Screws should be used to attach boards, because if you need to replace parts of the fence, the replacement will be easier.