1. Register/Login to your account from here.
2. Visit my account page from here.
3. Here click on View Licenses under Purchase History.

   

4. Here click View Upgrades and upgrade to your preferred plan. Once an upgrade has been chosen, you will proceed to the checkout screen where you can complete your purchase.
   

There is a Time-Based Proration for the license upgrade explained below:

Example: A customer purchased a Single Site license for Product A for $89 on October 1, 2017. On March 1, 2018 the customer upgrades their license to 2-5 Sites which is $129. The price difference is $40 and there are 6 months left in their current subscription. The customer will pay $20 to upgrade. On October 1, 2018, the subscription will renew and the customer will pay $129 for Product A (2-5 Sites) and have an active license key until October 1, 2019.

Note: The expiration date of the license will not change; an upgraded license will still expire on the original expiration date