Hire a contractor with experience with log cabins if you are not knowledgeable about the construction of log cabins. This ensures the job is done correctly and you do not decrease the stability of the cabin.
Purchase a replacement for the log you will be replacing. Log home supply companies can help you locate a supplier to buy a log that matches your cabin. Make sure to know the length the replacement needs to be when ordering. Depending on the reason you need to replace the log, it may be the full length of the wall or only a segment of the wall.
Cut out the log which you are replacing with a saber saw. The saw blade should be able to cut through metal such as spikes or screws in the log wall.
Remove the log by cutting and pulling with a chain saw and wrecking bar. You will be able to remove rotting wood more easily than solid wood.
Insert the new log into the space you just cut out by lifting it and sliding it into place. This may require several people to accomplish. If your walls are tongue and groove style, cut off the top tongue from the new log so the end is flat.
Attach the new log to the remaining wall with plated deck screws, using a drill. This specific type of screw resists corrosion and easily drives into the wood.
Use a tube of log chinking and a sealant gun/applicator to seal the joints between the old and new logs with log chinking. You can buy log chinking from a log supply store. Many hardware stores sell different styles of sealant guns. Follow the instructions on the applicator/gun to insert the chinking tube and apply the sealant.
This sealant creates a tight seal to prevent dust, moisture or other particles from passing through your wall.
Stain the log to match the existing wall (if it does not already) and apply a wood preservative to the area. You can apply the preservative to the entire wall or structure, if you desire.