Answer
If you're not an admin, go to the Account Page and tap Leave Business in the Danger Zone, below Delete Personal Account. It's in the same place on the web, iOS and Android. HelperHat asks you to confirm once and lists what happens: you lose access to that business's conversations, contacts and settings, and an admin would have to invite you again for you to rejoin. Your personal account and any other businesses you belong to aren't affected, and customers aren't sent any message. Choose Cancel or the close button if you change your mind.
After you leave, HelperHat opens the business picker if you belong to other businesses. If that was your only business, you go where you would if an admin removed you, which is the screen for setting up a new business. If leaving doesn't finish, you'll see Couldn't Leave Business, you stay a member, and you can try again.
Admins don't see Leave Business. To leave as an admin, ask another admin to change your role in Manage Team Members first, then tap Leave Business. If you're the only admin, make a teammate an admin first, or delete the business instead from the Account Page on the web.