My son gave me his iPhone, unlocked. How do I change it to my Apple id

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

This will erase the iPhone so that it can be set up again as a new device using the Apple ID and Password that you want to use.

You son needs to be “scrubbed out” of the phone by following ALL the steps in this procedure:

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

Hint: Step 8 is particularly important.

Then, you can setup the phone as yours using YOUR AppleID.