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 page.

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.

Click this to be taken to a confirmation page. Here you'll need to choose to unassign, 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 a organization once you have left, but either an Owner or General member (if enabled in your account) can reinvite you to the organization.

Note that if there is only one organization owner, they will not be able to leave the organization. They must first promote another user to Owner in that person's user pane.