When you initially register/purchase the software, you will receive a username & password to download the full version from a password-protected section on this site. Keep that username & password!
Whenever a new version comes out, simply sign in to the Registered Users area and download the new full version (do not download the demo version). If you forgot your password, use the Lost Password Retrieval form on the Registered Users page to retrieve it. If your email address changes and you can no longer retrieve your password, contact me and I will update your email address in my online system.
Once you download the latest version, just install it right over the old version.