View Upgrades link will be displayed on the right side if your license key has upgrades available. Locate the license you wish to upgrade and click this link to see the available upgrade options.
The total cost of the license upgrade will be displayed next to each upgrade option. Click on Upgrade License to proceed to the checkout screen where you can complete the purchase:
Once you have completed the upgrade purchase, your license will be upgraded and you can now activate it on additional URLs.