How to remove Apple Account of owner from iPhone?

You'll have to ask the previous owner to follow these instructions:

Sell, give away, or trade in your iPhone - Apple Support (CA)

If you don't know who the previous owner was you won't be able to use the phone.

Hopefully you did not purchase the device from a third party, that you can no longer contact. If so, you were likely scammed. See this user tip:

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

"How remove owner Apple id from my iPhone: I have iPhone 8 Plus"

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Important:

Leave personal information off of these forums and all other forums. These forums are out there for virtually anyone to view. So, I have asked the Host to remove your reply's info. The Serial Number is provided. Once someone sees this information, they'll know to whom this device pertains, and have your information.