how to re-assign an iphone to family member

First: What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support

Then set it up as you would a new one: Set up your iPhone, iPad, or iPod touch - Apple Support

Follow these instructions first-->What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support

Then set up the phone as New with the new user's Apple ID. If they don't have one, create a new one for them.

You can then link the phone to your family group-->Family Sharing - Apple Support