Hi Malcolm,
Thank you for your question.
You can do it either remotely or while being physically present at the Host computer.
Remote un-register
Right-click on the connection entry in your Viewer address book, select the "Register remote Host" menu item, then the Unregister radiobutton and click OK.
Unregistering locally
Right-click on the Host icon and select Settings for Host. Click on the Settings button, select Enter license key, then select the Unregister option and click OK.
Hope that helps.