Removal of an Apple ID
How do I remove an old Apple ID from my phone if I don't know the password?
iPhone 7 Plus
Enter the Apple (iCloud) ID and click Forgot Password.
If necessary, if you are the original owner, take the device, the original purchase receipt with the serial number, and a photo ID to the nearest Apple Store.
or contact Apple ID Support. They will need to see the original purchase receipt/paperwork also.
Apple Support by e-mail or chat
Or fill out and submit this form.