You can upgrade your license directly from the plugin if you go to Newsletter => Account and click on the “Change Plan” button:
If you have white-labeled your license you have to insert your license key first.
On the following page, you can upgrade your license by inserting your payment details. Optionally you can choose to get another license as well.
If you have a VAT ID you should enter it by clicking on “Purchasing as a business”.
Once you have entered your payment details you have to review your order.
Click on “Pay & Subscribe” to complete the checkout process and get redirected back to the account page.