Whatever your reasons for canceling, Lyft does not make is easy. You will lose your mind trying to find a way within the app. There is no "cancel my account" option, or any way to contact Lyft within the app. You can only close your Lyft account or someone else's account by contacting the ride sharing company.
To contact Lyft to cancel your account, you can use the contact form at lyft.com, email them at email@example.com or call 855-865-9553. Drivers can also email Lyft or call to delete their account.