Swift Commerce Pro is an extension of the Free version, where both work together. You can download Swift Commerce Pro by following the steps below:
Step 1: Go and visit the Members area of Swift Commerce: https://swiftcommerce.io/members/.
Step 2: Log in to the members area with your email and password sent to you over email.

Step 3: Click on the “Downloads” from the left sidebar navigation.

Step 4: Click on the “Download” button that will let you download the latest version of Swift Commerce Pro.

After that, you can proceed to install this on your WordPress website together with the Free version.