To win the war on the Eastern Front the Germans should have eased their racist anti-Slav policies. If they had backed the national committees that were established by different ethnic groups (Russian National Committee, Ukrainian, Belarusian, Georgian, etc.) then they could have had a potential source of millions of Soviets fighting on their side. The total number of collaborators with Nazi Germany from the Soviet Union is estimated to be more than one million at the very least, with the highest estimate being around 2.5 million.
Because of Nazi racial policies, the Hiwis (short for Hilfswilliger, German for "willing to help") and other volunteers, like Andrey Vlasov's Russian Liberation Army, were hindered and were rarely used in combat (the Battle of Stalingrad was one exception, where as much as one quarter of the German 6th Army's strength was estimated to be Soviet citizens). Although a lot of German commanders praised Soviet volunteers for being reliable and good fighters, they were ignored by the Nazis. General Field Marshal Fedor von Bock requested to Hitler that they put together a Russian Liberation Army of, initially, 200,000 men. However, the head of the OKW General Field Marshal Wilhelm Keitel told him to stay out of politics and did not forward his request to Hitler.
On the few occasions that Soviet citizens were used in combat they proved to be very effective. The ROA (the Russian acronym for Russian Liberation Army) fought against the Red Army in early 1945. They fought well and were praised by Heinrich Himmler and Joseph Goebbels for their vigor and ferocity. The ROA was better known for later defeating the Germans, ironically, at the Prague uprising. They managed to defeat the Heer and Waffen-SS divisions and drove them out of the city, saving the Czech partisans, who were on the verge of defeat before ROA's intervention.
The point is that they could have most likely toppled Stalin's regime if they had a policy of treating the natives better and actually used the Soviet volunteers in combat more often and effectively (many of them were sent to the Western Front instead). If they had, then the majority of the Red Army would most likely refuse to fight and defect in mass numbers. Even when the Germans were losing the war, they were still getting Red Army defectors. But this would have been probably impossible or highly unlikely at the very least due to the Nazi policies.
The only way I could see this possibly occuring is if high ranking Germans like von Bock and other generals, or perhaps Himmler (who backed the ROA and convinced Hitler to form ten Russian divisions in 1944, but it was far too late by then) went behind Hitler's back and allowed the formation and training of groups like the ROA and the national committees earlier on in the war; and also gave orders for better treatment of Soviet citizens. But that would have been highly unlikely to happen.