You are going to have to replace the post with a treated post. Ensure your post hole goes down below the frost line, place the post in the hole and fill with concrete. Let the concrete setup and then reconnect your fence. My fence post have been completed this way some 15 years ago and no rot so far.
Doing your repair properly requires:
Temporarily brace-up your fence.
Pry/unscrew fence & post pole apart,
excavate post pole,
replace/back-fill post pole,
and re-attach fence & post pole.
The following items will be necessary to replace the rotted post: New treated post, small rocks, small amount of concrete and shovel. First un-tack the rotted post from the fence. Dig out the rotted post. Pour small rocks into the hole. Mix up the concrete. Install the new post and hold in place until set. Re-tack the fence.
It really shouldn't hurt anything if it is only one but you can change it and as far as stopping the rot all you can really do about it is dip the bottom of the posts in liqid tar before you put them in the ground hope this helps.
Sad but true there is nthing you can do other than to replace the rotted post. Once you have your new post, you can prevent future rot by applying a sealer to the posts. This will extent their life. Another solution would be to switch to a non wood post.
You can either remove just that post and replace it if it's not in concrete. If it is in concrete then it's much easier to just put a new post in right next to it. Make sure at least a couple of feet of the new post will be buried.
Borrow a post hole digger and replace that rotted post with a new post. Coat the section of the post which will be burried with tar and throw some gravel in the hole before you put the post back in.That will help with drainage.They do that process with telephone poles and you see how long they survive
Well this usually happens if the pole was not installed properly or if the rot was not treated. Luckily, there are some solutions to this. You want to replace the rotted portion. Measure the rotted length and cut it off with a saw. Then you want to take some fresh plate of wood and cover over the area.
If its rotted out pretty bad theres really nothing that can save it and it will only get worse. You can try staining it and spraying pesticides and water repellents on it but you would be better of putting a new post in the ground.
Get another post and replace it in the same spot taking care not to interfere with the other parts not adversely affected by the rotten post. It is good you also find out the cause of the rotting to avoid the same problem.