This question kinda reminds me of Cher circa 1980, dancing on the deck of the U.S.S. Missouri:
"If I could turn back time
“If I could find a way…”
As for me, it’d be Potsdam, Germany in July 1945 as Truman’s trusted aide before the signing of the Potsdam Agreement between the "Big Three: Churchiil, Truman and Stalin.
I’d forewarn Churchill and Truman of Stalin’s evil intentions, and encourage them to stand tough against the Soviets in the granting of any post-war territorial concessions to Stalin.
I’d especially tell Truman – whom history records as having been “under Stalin’s spell at Potsdam” – to not make any territorial concessions of former Japanese territories in favor of the Soviets, i.e., Korea and Indochina
Specifically, I’d tell Truman that it is in U.S. interests to immediately grant full U.S.-sponsored statehood at war’s end, in proxy, to Korea and Vietnam. I would then condition the Soviet Union’s further participation in post-War talks upon Stalin’s full acceptance of such terms.
Such an action would thereby thwart the Soviet’s meddling in Korea and France’s troublesome claim to Indochina, thus preventing the Korean War and the Vietnam War from ever taking place at the expense of millions of lives decades later.