You can activate your license in your MyYoast account, a customer portal where you』ll be able to find and manage all your Yoast SEO products.
If you don』t have an account yet or want to know how it all works, check out the MyYoast Quickstart guide to get you started. You will need to create an account with your purchase email, add your site and then connect the subscription to your site. You can then update your older plugin to the latest version.
If your plugin version is below 5.x, please manually update to the current version. You can then add your website and connect your subscription to it using the new My Yoast portal. Good luck!