If you could change one event in history,

[Mike 03]

9. Perhaps a 13 minute delay for Hitler and his entourage on November 8, 1939

It's so hard to pick but just off the top of my head I can't resist...

Delay Hitler's 1939 speech at the Bürgerbräukeller by a mere 13 minutes and potentially prevent nearly all of WW2. (Poland had already been invaded). Without a war in the European theater to distract the United States, it's unthinkable Japan would have bombed Pearl Harbor.

Hitler ended his address to the 3000-strong audience of the party faithful at 9:07 p.m., 13 minutes before Elser's bomb exploded at 9:20 p.m. By that time, Hitler and his entourage had left the Bürgerbräukeller. The bomb brought down part of the ceiling and roof and caused the gallery and an external wall to collapse, leaving a mountain of rubble. About 120 people were still in the hall at the time. Seven were killed (the cashier Maria Henle, Franz Lutz, Wilhem Kaiser, a radio announcer named Weber, Leonhard Reindl, Emil Kasberger, and Eugen Schachta).[5] Another sixty-three were injured, sixteen seriously, with one dying later.[3]

Accompanying Hitler that night were Goebbels, Heydrich, Hess, Himmler and others.

https://en.wikipedia.org/wiki/Georg_Elser

It's a bit of a roll of the dice since we can't know for sure Hitler would have been killed, but you said we could change one event, so I'm changing it to "Hitler and his entire entourage were killed."