Convince everyone to try and truly understand their enemies (or people that hate them,or people that they hate).

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.

This is the smallest change I can think of,every thing else looks like a much bigger problem.

That is thoughtful and so true, experienced that myself. More people need to be made aware of that!

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.