How to turn off Two-Factor Authentication on iPhone?

You dont, sorry.

See this note (extracted from this support article:--> Two-factor authentication for Apple ID )

If you're already using two-factor authentication

with your Apple ID, you can't turn it off. If you updated to two-factor

authentication inadvertently, you can turn it off within two weeks of

enrollment. If you do, your account is less secure and you can't use

features that require a higher level of security.

Regards

Giulio