Do you mean how do you downgrade from Business Class?
This page can take you through the process - specifically:
Downgrade from Trello Business Class
To downgrade from Trello Business Class, you’ll need to be a team administrator. Either navigate to your team's billing page (https://trello.com/your-team-name/billing) directly, or navigate to your own profile and click on your team's name and the Billing tab.
On the Billing tab, scroll to the bottom of the page. Click the “Cancel Business Class” link to downgrade from Business Class and prevent your card from being billed in the future.
You can also contact https://trello.com/contact#/ directly to ask about billing support.
Don't get confused by the Ask Question page, that's just to ensure you've gone through any relevant Help pages, to get in contact with actual support is at the very bottom of the Help Page results.
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