First, make sure the free version has been deactivated and deleted.

Order Confirmation Email
Open the confirmation email sent to you and note the API License Key and API License Email, you will need these later.
Next, click the ‘Download Files’ link and log into your account page on Night Shift Apps.

After logging in, find the section MY API DOWNLOADS, then locate remarketable Pro and click ‘Download’ and save the zip file to a handy folder on your system.
My Account Download

Once you have downloaded the file, return to your WordPress admin section and select ‘Add New’ from the Plugins menu.  Click the Upload Plugin button at the top, locate the zip file you downloaded early and install and activate the plugin.

Activation Notice
Once the plugin is activated, you should see a notification at the top of your admin area asking you to activate.  You can also reach the activation page by going to remarketable Pro settings and clicking the ‘Activate Pro’ tab.

Now enter the email address and key obtained from the email earlier and click save.

Activate Settings

You should now see a confirmation message like the one below indicating you are ready to go!
Pro Activated