"What if you could change one thing in history, what would it be?"
Nothing on the premise who knows what the out come would be, and how much greater damage would have occurred, that would have had even worse results than what has happened. The possibilities of far greater results are infinite, and we can not project what those results would been. Altering one event would have altered other things, and the results could be far more devastating than what we could even begin to guess.
I watched Star Trek (the original series as a child), and a few episodes showed a potential outcome, where one small incident was changed in human history. The end results were worse. For example stopping Hitler, may have resulted in a far more vile, and evil person rising to power, who would have done even greater evils.
Stopping the evolution of man may have resulted in another sentient life form arising,that would have been even more devastating. So here again, the plausible possibilities are infinite. One must consider the affect any change, even the smallest could have on the mechanics of temporal physics of this space and time continuum would have, or would have had. It would mean having to constantly changing, and the altering of other resulting outcomes.
Therefor I would change nothing in history as I stated at the beginning. As I was preparing the end of my answer, I read through all of the other answers, just prior to posting. I maybe wrong, but then we will never know since we don't know what the resultant change(s) would have brought. Whether as individuals, or the result of each change. Could things have been better, yes, but then too, yet then too they could have been far worse.
Now I'll close with a couple of quotes I heard many years ago:
"I am not wise, for there are many acts of foolishness I have committed." Author unknown.
"I'm not wise, for I know how foolish I am, and have been. Therefor any who believe I am wise, are mistaken" Author unknown.