If you have reached the license  activation limit  there  is an option to deactivate any site that you do not want the plugin registered to, this way it will open up a license that you can use to register a new site. To register a new site , Please add the activation key to the plugin on the new site. 

To Deactivate Sites Login to your account in https://www.wpbackitup.com  then go to Account Section >Click License Keys  and there should be a Manage Sites link that will show all the sites that are already registered and there is an option to  Deactivate Sites.

This is what you should see in your screen.