What do you mean? I have the chance to change the world every day!! Just by existing, I am using up Oxygen and food and water, and I am taking up space and interacting with family, friends, and others, who in turn react to me and act just a little bit differently throughout their day because of me.
Now if you mean changing the world with limitless power...
I would found tons of charities and get other people to run them just so they exist, and I would get engineers to start working on all of the invention IDEAS that I have (that will never truly be realized due to other interests overruling it), and then whichever ones work and whichever ones are worth having in our society will be used by people. I would also make it so that the government is better with dealing out justice to criminals; rather than just punishing them and feeding them tax-payer-paid food and things, the program should work to try and change their heart (figuratively).
I think we could learn a lot from Hawaiian conflict-resolution tactics. We should use something like those. And I would make it so people in the U.S. drive on the left side of the road (We READ from left to right, so why not drive from left to right?? And yes I am an American.). If you want a more vague answer that doesn't have an actual action plan but is more of just an awesome fantasy, then I would start a media trend where everything- movies, books, TV shows, video games, magazines, etc.- is moral and clean, rather than full of crude language and scenes that people would be totally embarrassed to explain in front of a 10 year-old.
So yeah. Make things nicer, more Utopian. :)