Kill Stalin on May 9th 1945 and replace him with someone who wants to coexist with the West. Beria is the obvious choice, but arguably not the best. He was so hated by the other elites that his rule would be unstable without further purges, and in the process might discredit "Berianist" coexistence the way Stalin discredited, well, Stalinism.
The new leadership, whoever they are, avoids the provocations in Iran and Berlin. When the US makes its proposal for atomic disarmament under the UN, the US and the USSR cut a deal. The outline of the US plan was phased introduction of international inspections and control, with all atomic energy to be controlled by an international corporation owned by the member states, after which the US would dismantle our own stockpile. The Soviet Union, understandably, wanted the US to dismantle our bombs first before they agreed to inspections, but I've been wondering if they couldn't find a compromise.
Even without Stalin, it would be a slim chance, but it might be possible. For example, the US could agree to stop building new weapons during the phase-in, or to dismantle our bombs but keep the plutonium so we could rebuild them quickly. The USSR, at least publicly, always claimed to be in favor of the overall scheme but wanted a guarantee so that the US couldn't use the phase-in as a chance to arm themselves, and then break their promises when they came due.
With international control of atomic energy and a US-USSR relationship that may be chilly but is not openly hostile, there's no incentive to build ICBMs. Without ICBMs, early space exploration is a more leisurely affair, but it still occurs because both the powers and the international control authority will want reconnaissance satellites to look for violations. But it isn't a race, and with cooperative butterflies, the first moon landing is a joint US-USSR venture.