The information in this documentation should be correct.
Login to your server via ssh, and go to the conf subdirectory in your red5 installation directory
cd /path/to/red5/conf
There you should find a file called red5.properties
. Open it in an editor and change the port configuration according to your needs.
Don't forget to restart red5 afterwards in order to listen on the new port. You will also have to restart Plesk so it can bind to port 8443 again.