If you're no longer in need of all the good things that come with your paid subscription, you can downgrade to a Free Forever plan at any time.
You can cancel your subscription whenever you want, and here's how.
To access your Billing page from your account, click on your name at the top right of your home screen and chose "Billing":
Here, you can see which plan you are subscribed to and what features are included:
Underneath your current plan you have the option to "Cancel...." You can do so at any time and your account will be automatically returned to a Free Forever plan, where you don't have to pay a thing.
However, please note that as soon as you cancel your plan it will be reverted, and you will lose the resources included in your paid plan. But don't worry, right here underneath your plan information you'll be able to see the dates of your last and next payments so you can time it right and avoid this.