All maintenance updates are free. These are the updates between the major version numbers. So, updating from version 1.0.9 to 1.2.4 is free.
You'll need your original Purchase Receipt email to download the update. This email includes your personal download link. Let us know if you can't find the email. We'll send you a new copy.
Once you download the installer, just run it like the original. It will detect your current installation. The installer will not touch any of your imported TMs and engines. They are safe. If you have any questions, there is a discussion forum for installation, uninstallation, and updates here:
IMPORTANT NOTE: Our update to 1.2.0 included a change in our license authentication server. This means you'll need to manually uninstall any pre-1.2 versions, and all per-1.2 installers will not work. Follow the "Minimal approach" instructions in this article. Then, run the new installer.