Upgrading your plan couldn't be simpler. All you have to do once logged in is go to "Upgrade your plan" and then you should reach a screen that looks like the image below.
Once you see this screen you can click buy on any of the plans to upgrade your plan to that plan instead. For example in the image above, I can choose from Basic, Premium or Ultra. The plan you currently have will be easy to distinguish without a buy button like above. Hopefully this information will help you upgrade your account with ease.