If you cancel your Pro Subscription, it will only cancel the auto-renewal. You will keep all of your Pro features until it expires. Once it expires, your account will revert back to a free account and you will not be charged again unless you re-subscribe. You will not lose any of your Projects, only the Pro Features that you had.
To cancel your Pro Subscription, from your Avatar menu on the top right, go to:
Pro Subscription > Cancel Auto-renew