Keep free version installed on your website before you install the pro version.
Login to your WP CTA PRO account here: https://wordpressctapro.com/my-account
Once you are logged in, go to the API Downloads tab and click on the Download text.
In your user dashboard on my account page, you will see a tab called “API Keys”. Go to this tab and copy the license key and product number on a notepad or somewhere you can paste it on your site later.
Now login to your WordPress website and go to the plugins tab. Once you are there click add new plugin and then select upload. A file uploader will show and you can drag the file to upload it to your site. See images below.
Now go to the left hand dashboard, go to the WP CTA tab. In the submenu there is an activation page. Go to the activation page and enter the api key and product and then click save.