This is a powerful question. The ability to waive a wand and vanquish one ill from the world. I must have written 5 different answers to this question. If I had to pick one thing to change I would eliminate fear. I feel we are missing out on living up to our truest potential as a species because it is so easy to distrust and hate what we don't know, understand, or fear. We have been led to accept so many things as inevitable that humanity already has the tools to change. Whether it be education, poverty, hunger, etc. But too often we can let the fear of "the other", fear of not having, fear of losing face, fear of being wrong, etc get in the way of progress.