You can see an "Upgrade to premium version" option displayed in the settings of the free plugin page. This will take you to the product page of the premium plugin from where you can purchase the plugin.
Once you purchase the plugin, you can login to the ThemeHigh Account with the email ID and password created during checkout. You can download the premium plugin from the Downloads section in My Account page.
Please don’t delete the free plugin before installing the premium plugin. When you install the premium plugin, the settings of the free plugin will automatically copied to the premium plugin. After that you can deactivate the free plugin.