There is an option in your online dashboard to change your email address.
Just visit the following page: login.pcprotect.com
Login with your current email address and password. Go to "Account", change your email address and "click save changes".
You may find that a few minutes after changing your email address, your app reverts to a free version. If that happens, just log out from the app (go to settings, scroll down and hit the logout button), and log back with the new email address that you've just changed.
Please note that the app may automatically start a "Quick scan" again.