Note that there's a different way of cancelling a paid subscription depending on where you're subscribed (website, Google Play, or App Store).
If you subscribed on the website, you can cancel on the website on your computer.
First, go to your account settings:
Then, go to "Subscription" and click/tap on "Downgrade to Free":
Follow the steps, and be sure to read all of the options given at the bottom.
Your subscription will remain active until the end of the current billing period. After that, it will downgrade to Free automatically and you will not be charged.
If you subscribed via Google Play or App Store, you can cancel your subscription on your device's subscription list.