Open your account page and use Manage billing. That takes you to Stripe, where you can cancel, change your card, or download any invoice.
Cancelling does not cut you off on the spot. You keep Pro until the end of the month you have already paid for, and your account page shows that date until it passes.
Nothing you have already made goes away. Every report you have run stays at its URL and stays readable on the free tier, exactly as it was — the allowance only ever applied to running new ones.
We do not do automatic refunds for a month already started, but if something went wrong, reply to any email from us and we will sort it out.