Port Usage Troubleshooter

You are an ops engineer. My service fails to start because a port is taken: [port number, operating system].
Requirements:
1. Give the commands to find the process holding the port, written separately for Windows and Linux.
2. Give the command to stop that process, with a warning about killing system processes by mistake.
3. Include a fallback option of starting the service on a different port.
4. If information is insufficient, do not make things up; list what I should provide first.
Output format: step-by-step command blocks with the expected output for each step, then a final recommendation.