ThinVNC listens on port 8081 by default. If you are not using this port yet it won't be necessary to change the ThinVNC port.
Validate whether ThinVNC is running well by looking at the status message of the "General" tab, located on the bottom of the window. It should say:
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If you see the message "HTTPS port 8081 in use", it means that you will have to use another port number, since this one is already in use by another application.
1. Identify a port number that is not used yet in the computer where you have installed ThinVNC.
2. Change the port number on the ThinVNC Server Settings "Communications" tab.
3. Press "Apply".
4. Verify whether ThinVNC is running in the status message of the "General" tab, located on the bottom of the window. It should say "Server started. Listening https on port...".