Assessment: Inspect the log to determine the extent of rot. Check for structural integrity by tapping on the log and looking for soft spots.

1. Determine Replacement Type: If the log is primarily for aesthetics, a face replacement may be sufficient. If the log supports load, structural replacement..

2. Removing the Old Log: Carefully cut out the rotten log using a chainsaw, making sure to avoid damage to adjacent log sections. Use pry bars to extract the log without impacting structural components.

3. Treat Surrounding Area: Inspect adjacent logs for any signs of rot or damage. Treat with a preservative as needed to prevent future issues.

4. Installing the New Log: Cut a new log to match the dimensions of the old log. Secure it in place using lag bolts or construction adhesive, ensuring proper alignment with the existing structure.

5. Finishing Touches: Seal any gaps with caulk or foam insulation. Apply stain or finish to match the surrounding logs if necessary.