TURNING off TWO-FACTOR AUTHENTICATION

See the following instructions from Two-factor authentication for Apple ID - Apple Support

Can I turn off two-factor authentication after I’ve turned it on?

You can't turn off two-factor authentication for some accounts created in iOS 10.3 or macOS Sierra 10.12.4 and later. If you created your Apple ID in an earlier version of iOS or macOS, you can turn off two-factor authentication.

Keep in mind that when you turn off two-factor authentication, your account is protected only with your password and security questions.

To turn off two-factor authentication, sign in to your Apple ID account page and click Edit in the Security section. Then click Turn Off Two-Factor Authentication. After you create new security questions and verify your date of birth, two-factor authentication will be turned off. If you don't see the option to turn off two-factor authentication on your Apple ID account page, that means you can't turn it off.

If someone has turned on two-factor authentication for your Apple ID without your permission, you can also turn it off from the enrollment confirmation email sent to your Apple ID or rescue email address. Click Turn Off Two-Factor Authentication at the bottom of the email to restore your Apple ID to its previous security settings and regain control of your account. The link is active for two weeks after you enroll.

That is only if someone turned it on without permission. Use the link below to turn it off.

Apple ID - Manage