I'd stop the Cold War from ending. That single 40-year period birthed so many technological achievements. Imagine where we would be today if it went on a little longer WITHOUT the conflict actually coming to full-scale arms.
(obvious rebuttal is obvious : we'd all be dead/super mutants)
Out of sheer fear, humans created, in one generation: the computer, silicon, space-travel, nuclear energy, and countless other innovations. While today we have some amazing technology, it has only been through implementation and refinement of the very things created during the Cold War.