Do you have Pro edition installed now?
If not your purchase history should show how to upgrade.
If there is nothing there to help then go to Change product key in Settings > System > Activation and enter the default Pro key VK7JG-NPHTM-C97JM-9MPGT-3V66T. (make sure you are offline while you upgrade) you may have to restart but once the upgrade is complete and you are back online it should activate using the licence you bought.