By upgrading you pass on to another plan with a lot more functionalities. In order to upgrade to a plan, first you need to login to your account. Then, click on “View Upgrades”.
Then, follow the “Upgrade License” link and proceed to checkout. The amount you will be charged to upgrade is based on the price difference between your current plan and the plan you are upgrading to, and the time left in the current subscription before it renews.