Apple ID B.S.

1. You can't turn this off once it's been enabled for two weeks.
2. You should be able to do this an https://appleid.apple.com
3. If you don't have a separate address from your @icloud.com address and something goes seriously wrong with your account Apple will have not way of contacting you, or sending you a reset password.
4. You do this at the address above for the ID itself; iCloud details are amended at https://www.icloud.com on a computer.

From the article below updated on 8 Nov 2019. If it has been on longer than 2 weeks, you can't turn it off. 

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

If you already use two-factor authentication, you can no longer turn it off. Certain features in the latest versions of iOS and macOS require this extra level of security, which is designed to protect your information. If you recently updated your account, however, you can unenroll for a period of two weeks. Just open your enrollment confirmation email and click the link to return to your previous security settings. Keep in mind, this makes your account less secure and means that you can't use features that require a higher level of security.

Apple ID -Two-factor authentication

Log into your Apple ID to modify Reachable At. You can't remove/change @iCloud.com, @me.com or @mac.com aliases. If there is an X on the right side of the address, you can delete it.

Apple ID - Manage