I'd be too afraid to change anything for fear it would, somehow, make things worse.

I do not dwell on impossibilities.

The United States stays neutral in World War I and does not enter the war. Had we stayed out of the war we would not have been in World War II. Communism would never had won out in Russia. there would not have been a cold war etc

Stop that creature that became human existing.

Documented history: probably the birth of Adolph Hitler; he would not have been.

Evolved history: the rise/development of surplus agriculture.

Well possibly the Death of Charlemagne would have probably stopped the spread of Christianity over all of Europe and in turn the United States. But that void would’ve only been filled by something else maybe even worse. Maybe find a way to spread Jainism across the world in 100 bc. That would be interesting.