How can I turn off 2-factor authentication on iPhone?

I see you want to disable two-factor authentication. For some background I suggest you read the document: "Two-factor authentication for Apple ID" - Two-factor authentication for Apple ID - Apple Support

Some may find the article somewhat difficult to follow and some wording unclear so I am providing what I feel is clearer information. When presented with isolated quotations from that document it is easy to misunderstand if you can or cannot disable two-factor authentication. Hopefully you will make it this far in reading replies to your question! This is how it works:

Several years ago Apple transitioned to using two-factor authentication for additional account security. Accounts created since that time automatically use two-factor authentication and this cannot be disabled.

Some people have older accounts from before two-factor authentication was mandatory on new accounts. Apple encourages those users to transition to two-factor authentication when signing in on their accounts because without it "your account is less secure and you can't use features that require a higher level of security." Those owners of older accounts who just activated two-factor authentication have two weeks to change their minds and return to older security settings.

If you have an older account, recently activated two factor authentication, and you're within the two week period for turning it off again, you should check for an email from Apple for a way to do this. If you don't see one about this, look in your junk mail folder.