How to sign out of Apple ID on iPhone?

I got an iPhone from my grandpa, but he has forgotten the Apple ID password. Is it possible to sign out of this Apple ID and use my own?

How to sign out of Apple ID if I forgot the Apple ID password?

If you forgot the Apple ID password, you can't sign out of Apple ID. The simplest way is to reset the password. Click the question mark icon to reset the Apple ID password. If the Apple ID is correct, tap Next.

Click Reset by email and you will receive the password reset email. Open the email and follow the directions to reset your Apple ID password.

You can also choose to answer security questions to reset your Apple ID password.

If you neither have access to the email address nor remember the security answers, you can't reset the Apple ID password. The only way to sign out of Apple ID is to erase your iPhone and set it from start.