how to turn off Two-factor authentication?

Two-factor authentication for Apple ID - Apple Support

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. Use the Unenroll link in the email you received when you turned it on.