To deactivate a Parallels Remote Application Server farm, log in to your Parallels account.
Open the Remote Application Server menu and go to the appropriate Farms page depending on what license you use - with post-paid or pre-paid billing.
You will see the list of the farms.
Click on the one you need to deactivate and you will see the Deactivate option, click on it.
Now you can use your license key for activating another Parallels Remote Application Server farm.