I would step into my "Way-back Machine" find the exact moment when it was decided, and by whom, to bring people from one country to another as slaves. At that moment I was eradicate the source.
If you could insert yourself into one point of history to change the conversation, to affect an outcome, when and what choice would you make?
While everyone else is off changing the world, I think I would be a little selfish and go back in time about a decade and tell myself "Don't do it! It will have devastating consequences!"
I would go back and talk to Constantine.
Stop the Council of Nicea from deciding on "one god" ... And change it to endorsing religious freedom and tolerance.
Maybe ... Just maybe ... That would have stopped hundreds of years of religious persecution and wars.
I don't think there is ever really one moment that can that effect a major change. Most historical events are based on a series of moments and general principles. Talking to one person would change little in the grand scheme of things.
On a personal note I think I would have talked to both my grandmothers about the importance of mammograms. They were both diagnosed in the 80s with breast cancer. This is the time when mammograms were really just becoming part of annual exams and they never had one until other symptoms arose. Who knows how much longer they would have lived if it would have been caught earlier. They both had parents that lived in to their 90s.
I would like to go back to the first guy to sag his pants and hand him a belt.
I would pop in on Jesus Christ about 6 months prior to the crucifixion and convey my strong preference for the concept of disambiguation.
Stop the Egg and Sperm donors from, (I'll be kind) making me. A double barrel shotgun blast twice to both their heads.