If you have a trial license, you must revoke it first by login in as an admin and selecting Setup => License, where you will see a Revoke button.

Alternatively you can delete or rename the asterniclic.php file located into the Call Center Stats PRO web root directory, usually located in /var/www/html/stats/asterniclic.php.

Then browse the software main page and it will ask you for a registration code again, put the one you received via email when you bought the license.