After punching the premium version of the plugin, you will be able to download the zip file of the premium version. An email with the download link will be sent to your email address. Also, you can download the latest version of the plugin from your account page.
Now go to your site admin area > Plugins > Add New > Upload Plugin. Here upload the downloaded zip file and activate it. The free version will be deactivated automatically. You can delete the free version if you want.