Convince everyone to try and truly understand their enemies (or people that hate them,or people that they hate).<p>Whether you want to resolve your differences or destroy each other,don't do it without understanding your opponents and their motives. It's too easy and tragic to wrongly presume a person's motives and beliefs.<p>This is the smallest change I can think of,every thing else looks like a much bigger problem.
I would change myself, make myself a vastly more intelligent, good looking, lively and spontaneous person,
and yes I will throw in immortality to the mix, I will so to speak rewrite my genes to become a perfect being so to say(I now wonder if perhaps according to the grandfathers axe paradox the resulting being will still be me), then I will proceed to try to change the world, having lovely fun along the way.