If you run the update and let it download all available updates first, the option to enter the license will appear. Might require a restart unless you close Start Center while updating. Simply click on the "Free Edition" in Start Center, and the opening license dialog will have the button to enter the license key.

You can also use
https://www.safer-networking.org/lost-license/
To retrieve your key if it has gone missing along with the license installer.