Warning shots are almost always a bad idea. You are responsible for the final resting place of each and every shot you ever fire. That shot you fire up in the air will come down somewhere. That shot you fire into the ground may ricochet. So you greatly risk hitting an innocent with your "warning shot."
Furthermore, you may well need that shot later.
If you are not justified in using deadly force, then you shouldn't shoot. If you are justified in using deadly force, you should shoot at the upper center of the chest, with a secondary target of the head if shots at the chest don't stop the attack.