Everything happens for a reason. It's a very cliché thing to say, but it's true. If you wanted to go back in time and change anything that has happened, I personally wouldn't suggest it.
When you got a bad grade or made the wrong decision, it was for the best. You wouldn't be who you are today if you hadn't done those things. Every small change you make to the past can drastically change the present for you and everyone around you. There is a movie called "The Butterfly Effect", which I found very accurately depicts what would happen if you changed even the slightest thing about your past.
Let's say you went back in time to kill Hitler when he was a baby. The World Wars never would have happened and we wouldn't have lost so many Jewish people. However, the World Wars are the reason so many things that we have today were invented. The loss was great, but it was needed. I'm not condoning what happened; I'm simply stating the facts here.
If you could go back in time to change something, what would you change? Why? How do you think it will effect our present time? If you wish to, please comment your answers.