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.
