How to replace account already sign in on 2nd hand iphone 7?

If no refund is available from the seller, and if the seller is unwilling or unavailable unlock the iPhone for you, it will never be useful to you. Recycle the iPhone. Activation Lock is intended as a theft deterrent, and this iPhone is indistinguishable from a lost or stolen iPhone, if not actually lost or stolen. We can’t help. Absent proof of ownership you almost certainly don’t have, Apple won’t. Yes, really, recycle it.

https://discussions.apple.com/docs/DOC-250001038

Activation lock is a security protocol that can't be bypassed. Only the person it is locked to can unlock it.

If you bought this from someone and it came locked, try to get your money back, you will never be able to use the phone.

If your phone is Activation Locked - How to remove Activation Lock -

How to remove Activation Lock - Apple Support

The All Too Common SAD Reality of Buying a used phone -

The All Too Common SAD Reality of Buying … - Apple Community