How can I unlock an iPhone locked to the owner?

You need to have the owner's Apple ID and Password.

Do you have this information? If not, the phone is useless to you.

How to remove Activation Lock - Apple Support

Did you buy a used iPhone, which is locked to owner? Unless you know the owner and they are willing to remove the phone from their account, you own a useless iPhone you can't use and Apple won't help you unlock.

Read this if you bought a used iPhone --> The All Too Common SAD Reality of Buying … - Apple Community