Leave your organization in Flow by visiting the People page and clicking your avatar. You'll be presented with the option to leave, and asked to confirm.
If you are no longer working with an organization and want to keep your Flow account up to date, you can leave the organization on the People & Teams page. To get there, click the organization icon in the top left and then click People & Teams in the dropdown.
Once there, click your own avatar to open up your user pane. Below the list of teams you belong to, you'll see a red "Leave organization" link.
Note: If you are the only organization owner in the account, you will not be able to leave the organization. You must first promote another user to Owner in that person's user pane
Click this to be taken to a confirmation page. Here you'll need to choose to un-assign, complete or reassign any open tasks in the teams you belong to. If you don't have any open tasks in the team, it will say so below the team name.
Once you've chosen what to do with your tasks click the "Leave organization" button at the bottom of the pane.
There is no way to re-join an organization once you have left, but either an Admin or Member (if enabled for your organization) can re-invite you to the organization to restore your account.