Go to the web Websites section of your Freemius Dashboard. Click on the row of the website that currently has the license activated. If there are multiple products running on that site, the next screen will show you all of those products. Simply click on the relevant product to open the details panel, where you’ll be able to click the Deactivate button under the LICENSE section.
After the license is deactivated, you’ll be able to reuse it on any other website.