How to turn off two-step authentication on I-phone.

Just to clarify, there's 'Two-step Verification' and 'Two-Factor Authentication'. To which are you referring? If the latter, then as Lyssa said, you only have 14 days in which to turn it off. If the first, then:

How do I turn off two-step verification?

1. Sign in to your Apple ID account page.
2. In the Security section, Click Edit.
3. Click Turn Off two-step verification.
4. Create new security questions and verify your date of birth.

You'll get an email confirming that two-step verification for your Apple ID is off.

Two-step verification for Apple ID - Apple Support

If you're not sure which of the two you activated, see:

Switch from two-step verification to two-factor authentication - Apple Support

Confirm which feature you're using

If you're not sure if you set up two-factor authentication or two-step verification, use these steps: 

1. Sign in to your Apple ID account page.
2. In the Security section, look for two-factor authentication or two-step verification, then see if the feature is off or on. 

If two-step verification is on, continue with the steps to turn it off, then turn on two-factor authentication. If two-factor authentication is on, then you're already set up. 

If you've been using it for more than 2 weeks, you cannot. Otherwise, there is a link to un-enroll in the email you received to set it up.

~Lyssa