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. 

Please  login to your account on  https://www.wpbackitup.com   then  go to the account section of  WPBackItUp  This is  what you should see on your screen:


WP BackItUp View License

Select  View Licenses  there should be a Manage Sites link, click that and it will show all the sites that are already registered and there is an option to  Deactivate Sites.




WP BackItUp Manage Sites

WP BackItUp Deactivate Site/ License