Cancel Trello Gold
You can downgrade your Trello Gold account from within Trello. This will stop you from being billed again in the future, and will remove the option to use Gold features in your account. Cards and boards which made use of those features (like custom backgrounds or attachments larger than 10 MB) won’t lose them, but you won’t be able to upload new backgrounds or add new large attachments.
Downgrade from Trello Gold
To downgrade from Gold, navigate to your billing page. Scroll to the bottom of the page and click Cancel Trello Gold. This will cancel your Gold subscription, and prevent you from being billed in the future.