Access the 3CX Management Console
You should be able to do this from any web browser on your network by browsing to:-
http://1.2.3.4:5481 (replacing 1.2.3.4 with your 3CX server's IP address).
Login to 3CX using the account details you created when you installed 3CX.
Select the 'Activate Licence' option
From within 3CX, select 'Activate Licence' from the 'General' section at the lower left of the interface.
Enter your licence key
Enter your new licence key into 3CX (using copy and paste is recommended to avoid typos) and then click the 'Activate' button.