Two factor authentication
I want To remove two factor authentication, is it possible?? I want to sign in to iCloud on a iPhone 3GS
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You can't turn off two-factor authentication if it has been on longer than two weeks. See the "Can I turn off two-factor authentication after I’ve turned it on?" section of Two-factor authentication for Apple ID - Apple Support
Regarding the iPhone 3GS, lso see the following from that same link:
What if I use two-factor authentication on a device running older software?
If you use two-factor authentication with devices running older OS versions—like an Apple TV (2nd or 3rd generation)—you might be asked to add your six-digit verification code to the end of your password when signing in. Get your verification code from a trusted device running iOS 9 and later or OS X El Capitan and later, or have it sent to your trusted phone number. Then type your password followed by the six-digit verification code directly into the password field.