In order to change the license key in v15 you will need to reinstall 3cx Phone system. As the FQDN will be bound to the license key you will need to log in to the customer.3cx.com use the credentials located in the email you received with the PBX edition key and release the FQDN from the key in order to use the same FQDN again with the new key. See this guide for more details: http://www.3cx.com/docs/fqdn-already-in-use/ .
Kindly note, in case that your FQDN ending with 3cx.us domains are actually exhausted and can no longer be used.
You will need to choose a completely different 3CX FQDN i'm afraid. One like north.3cx.us so your FQDN will become xxxxx.north.3cx.us.
Please find the below how to proceed:
- take a backup that does not include FQDN or license (option to untick in the backup popup)
- save the backup zip file on your desktop.
- uninstall this 3cx phone system
- install 3cx phone system
- restore when prompted in setup wizard stage.