Upgrading from one product to another or between different levels of the same product can be done from your Account. Once you are logged in navigate to your purchases tab or your licenses tab. If there is upgrade pricing available then you will be able to click "View Upgrades" to get a list of your options.
The cost is prorated. You will pay the difference divided by the amount of time you have left before it renews. From then on your license will be able to be renewed at the normal price for the upgraded license.
Make your selection, and check out as normal.