Well, in order to answer that question I think you first need to look at why the Song lost to the Jin in the first place, and why they failed to reconquer the north OTL. One major obstacle was the imbalance in cavalry. Historically, horses suitable for cavalry haven't been native to China proper, so dynasties had to rely on either tribute from nomadic states or their own conquests in Central Asia. Post-Tang no dynasty until the Qing had holdings in Central Asia, and the Southern Song had no tributaries with horses, either. This was a huge handicap in favor of the Jin, which was only compounded by military overconfidence and incompetence on the part of Song. If you want Song to defeat Jin, you could start by giving them a few more competent generals who see how militarily successful Jin is during the war the two fought against the Liao, and which caused them to become enemies in the first place. Also, the Song might avoid, or at least delay, conflict with the Jin by not being so bullheaded about the division of Liao territory, as that was a major sticking point between the two. Overall, however, I think the main factor was what has been a huge problem for basically every native dynasty after the Tang, military incompetence. The Song had a very large and modern army, but they mismanaged their resources, didn't have many skilled generals, and they put far more focus on things other than the military, in that the elite was very Confucian, and so placed little stock in or respect for the usefulness of the military.