If can upgrade your license from your account.
Log in to your account by using your email and your password.
Click on the “View License” link and then on the “View Upgrades” link.
You will see a table with the possible upgrade paths (if they exist). Chose the one you want and click on the “Upgrade License”.
You will be redirected on the Checkout page and you will pay the difference between your current license and the upgraded one.
Once the payment is processed, your license will be extended.
You will get a new Purchase Confirmation email, just like you did for the initial purchase.
If the new license contains new products (possible for some bundle upgrade paths), you will need to install them.
If your new license is related to the same product and it’s just extending the number of domains, you don’t need to do anything extra. You will be able to activate your plugin on new domains.