"I'd probably saaaaay....
seeing to it that Adolph Hitler's mother got an abortion."
Too late, to do so sooner would be effective to a point, but now you lose more than you gain. You should abort a problem of the day, I would choose myself.. Though such is not wise as no future can ever be had if any attempt it, it is clear it would continue, also they would often prove themselves wrong to do so.
So it is impossible, even if it is merely impossible. As far as Humans should be concerned... The instant it happened, there was no question to be had. Likely too, the past which vanished twice, may not have been past at all to those altering it, but some other form of structure touched and changed.
Which leaves myself pondering which aliens killed the dinosaurs and how advanced was their first attempt, which we see the only part of in fossils.
They may have been wonderful, but for the few wrong. ??