Is there a way to change the default port for the urbackup web interface (not the internet server)?

I just want to customize it to something a little more secure than the default port (55414).

I’m using a CentOS 7 server.

Sure it is possible adding --http_port directive to start_urbackup_server wrapper. You can edit systemd unit and add environment variable. But it is “Security through obscurity” approach.
The best way to make it really secure is to use external webserver as it described here
and add SSL (HTTPS). Also, you need to deny external connections to 55414. Maybe it’s time to add it to manual (with nginx config example).

Stop service UrbackupServer
Edit the file c:\Program Files\UrBackupServer\args.txt
search 55414, replace to 80
Start service