By default the plugin starts a MQTT broker on port 1883. If you want to use an external MQTT broker instead, enable External, set the IP address and the port of your broker and optionally a username and pasword, then save and restart the plugin.
If you want to secure the internal broker with a username and password, set those fields, save and restart the plugin.
If you just want to change the port on which the internal broker is started, set the new port, save and restart the plugin.