You can update from Free to Pro at any time by tapping the [Upgrade to Pro] button, choosing a Plan, and entering your credit card information.
There are four available Pro Plan types:
Note that you can pay by the month or by the year, with a discount for paying yearly.
To downgrade from Pro to Free, simply go to the "Billing" tab under the gear icon to the upper right of your dashboard. You will find a [Downgrade to Free] button there.
Because Free accounts allow only one app, if you have added multiple apps to your Pro account, you will have to choose which of the apps in your account you want to keep.