Two-factor authentication

If you turned it on less than 2 weeks ago, you use the link in the confirmation email you got when you activated it to turn it off.

If it's been more than 2 weeks, or you enabled it when you created your account, you can't turn it off.

If you have had it less than two weeks you open the email you received when it was activated and click on the link to deactivate it.

If it has been more than two weeks you cannot turn it off.

You cannot.

See the next-to-last question near the bottom here.

Two-factor authentication for Apple ID - Apple Support

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