How I unlock the activation lock

You need the Apple ID and password that were used to enable Find my iPhone on that phone. If you don't have those items and can't get them the phone cannot be used. You must contact the person who knows that information. This is what they have to do-->What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support

Find My iPhone Activation Lock

Find My iPhone includes Activation Lock—a feature that's designed to prevent anyone else from using your iPhone, iPad, iPod touch, or Apple Watch if it's ever lost or stolen. Activation Lock is enabled automatically when you turn on Find My iPhone.

Activation Lock helps you keep your device secure, even if it's in the wrong hands, and can improve your chances of recovering it. Even if you erase your device remotely, Activation Lock can continue to deter anyone from reactivating your device without your permission. All you need to do is keep Find My iPhone turned on, and remember your Apple ID and password. And with watchOS 2 and later, you can use Activation Lock to protect your Apple Watch too.

Find My iPhone Activation Lock - Apple Support

https://support.apple.com/en-us/HT201365

 How I unlock the activation lock