If you cancel your Premium Subscription, it will only cancel the auto-renewal. You will keep all of your Premium 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 Premium Features that you had.
To cancel your Premium Subscription, from your Avatar menu on the top right, go to:
Subscription > Cancel my subscription