How do I disable two-step verification?

Per 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."

If it's been less than two weeks since you turned two factor authentication on you'll need to go to the email you received saying it's been enabled and follow the steps in the email to turn it off.