well if you want to modify the port, you have to stop nifi, modify nifi.properties and restart nifi. But without knowing how you configured NiFi ( content of nifi.properties) nobody can tell you how to solve your issue. Maybe you can add the content of nifi.properties here and afterwards we can guide you further. Nevertheless, how Matt stated below, you need to check your application logs and see if NiFi started correctly. If NiFi starts correctly, you will see a log line saying that NiFi UI (User Interface) is available at a specific URL. You need to take that URL and paste it in your browser. If such a line is not available, than you need to check the logs to see what the error is.
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