to change your public IP in V15 navigate to settings >> Network and under the External IP Configuration change the value of your Ip address. Restart all services and if you have your own DNS server update your A record.
When the local IP address of the PBX changes you will need to perform a backup without including license and FQDN, un-install using apt-get --purge remove 3cxpbx and reinstall using your backup.