Change?

I feel change is a monumental thing in our society. It has helped many of us become quite good at adapting which is a phenomenal trait. What if, however, you were able to change the world? Anything you wanted to change in the world, but there is one catch. You are only able to change one thing. You don't get a billion changes. What would you change? Would you be broad or precise? Would you just get rid of all the evil in the world? If there is no evil though, can good exist? Just a simple thought exercise. I wouldn't mind a few comments with your thoughts.