How do I delete two step verification?

According to Apple:

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, you can unenroll within two weeks of enrollment.

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 higher security.

If you are using 2 Step Verification. 

How do I turn off two-step verification?

Sign in to your Apple ID account page.     

In the Security section, Click Edit.

Click Turn Off two-step verification.

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.