You simply open the application, switch to the License tab and follow the instructions.
You will be asked to enter your account credentials: e-mail and key.
If the Licensing Server approves the registration, you will be given a Recovery Password.
The registration is only successful after you confirm that you have saved this Recovery Password in a safe place by writing it and clicking "OK".
You simply open the RD/ShowCockpit application, go to the License tab and click the Un-Register button.
With respect to license recovery, there are three levels to it:
If you still have access to the licensed RD/ShowCockpit instance, you can navigate to the License tab and click on the "Un-register" button.
If you happen to be installing on the same computer with exactly the same hardware, you can just try to re-register the license - it may work and override the previous registration.
If you don't have access to the installation nor the system (or option 2 does not work), the only way to recover a license from an installation you have no longer access to is by using the Recovery Password (explained above). On your new computer, click on "Recover License" and use your account credentials and the Recovery Password for the previous installation.
After any of these 3 options, you can then register the license on the new computer.
Albeit the many warnings you will find along the way, there is the possibility that you lose both the Recovery Password and also access to the previous installation and/or system.
In this case, unfortunately, there is no way to recover that registration.
In this case, your only option is to add Extra PCs to your account through the Store Upgrade page.