beshir160 Said:
"How can I change my iPhone's Apple ID?: i have my own Apple ID but I can’t sign in on my iPhone 7plus because this my phone is signed in by other person Apple ID for first time (before three years) so how I can sign in and update my phone?"
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Troubleshooting a Purchased, Locked Device:
Getting into This:
You need to have this person disassociate it from their account. So, contact them back, having them disassociate this iPhone from account. This is the site they should have used: What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support. The seller should have known better.