Go to your Profile page → Billing → Manage billing. This opens Stripe's Customer Portal, where a Cancel plan option is available at any time, no need to contact support or explain why.
What happens after you cancel
Your plan stays fully active until the end of the billing period you already paid for, cancelling doesn't cut off access immediately. You won't be charged again after that period ends, and your account then automatically moves to the Free plan rather than being locked or suspended.
What happens to your projects
Nothing is deleted. See "What happens to my projects if I downgrade or cancel?" for the full picture, in short, your projects and their data stay exactly as they are, only plan-specific benefits (like a custom domain) stop applying once you're back on Free.
Changed your mind?
You can resubscribe at any time from the Pricing page, there's no waiting period or penalty for having cancelled before.