@HeatherO82 You need to login to your self serve account (use incognito/private mode) and then select Plans/Add-ons from the left side of page, then Change My Plan and scroll with the arrow to the correct $40 plan as there are two and then select Change at Next Renewal option as PM doesn't prorate for unused days left in your current cycle if you choose Change Now.
PM is also offering 20GB for $45 in self serve too.