Before removing your payment method, please make sure your plan has been successfully downgraded to Free.
Note that the downgrade does not take effect immediately.
Once your plan shows Free, go to the Plans and Billing page and click Manage Billing. This will take you to the Stripe page. Under Payment Methods, select Delete.
If the Delete button appears greyed out, your plan has not yet been downgraded to Free. If it appears red, you can safely delete your payment method.