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 anytime. Here's how:
To access your Billing page, click on your name at the top right of your home screen and chose 'Billing':
Here, it is displayed 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 you will be automatically returned to Free Forever, where you don't have to pay a thing:
However, you should be aware that as soon as you choose to cancel your plan will be reverted and you will lose the features 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.