To cancel your Whimsical subscription, go here. Alternatively, click on the name of your workspace (upper-left corner of the screen) → Subscription → Cancel Subscription.
If you cancel your subscription in the middle of your billing cycle, your subscription will be cancelled at the end of the billing cycle you have already paid for.
Your existing boards will not be deleted. You will still be able to view and edit them. The only thing that changes when you cancel your subscription is that you aren't able to create new boards.