The first step, you need to deactivate the Free version of the plugin and install the Pro version. After installing the plugin. Please, go to WooCommerce » Settings » Shipping » Shipping Zones. Then add Royal Mail Pro to one of the zones, then you can access the settings page by clicking on the added shipping method.
At the right of the settings page, click on Add/Update Your License.
You’ll be redirected to the activation page:
The page will ask you to fill in your license key, you can find your license key in your purchase history page, scroll down to your purchase and click on “View License”. An alternative is your purchase receipt email.
Copy your license key and paste it into the activation page and then click on “Save Changes”. The page will inform you that you successfully activated your license. Now, you can get back to the plugin’s settings page and configure it as you wish.