The Polish by 1939, are fucked. The Polish are in a bit of a pickle anyhow. Because of the land that they snuck from the Lithuanians after WW1, they have no relations to speak of with their most immediate Baltic neighbor. Due to continued disputes over territory, they have no ally to the south to count on. And to the East and West, they have two large, heavily militarized, nations eager to conquer the nation, and return it to its former state as conquered territory, be that by outright colonization, or puppetization, to the point where the national border is more de jure then de facto.
So from the get, Poland is just not in good straits. Relations with the French went down hill after the Sanation regime of Pilsudski, and the British had always been a bit apathetic to the well being and goings on of the Second Republic. That being said, the Polish military could have done fine, I believe against ONE of the armies. It is a given that Poland is not the favorite to win, but they do have a fearsome tenacity on the battlefield, which accompanied with competent leadership, can lead to vastly impressive feats. In 1920, the Bolsheviks were at the gates of Warsaw. By 1923, the Poles had regained most of the territory held in 1919, with the key exceptions of Minsk and the like. I believe that if Poland had been attacked by just the Germans, the war could have gone into a stalemate. If the Soviets had attacked, it could have gone just as well as it did in OTL, but I doubt they'll capture Warsaw. But with a POD in 1939, you cant change the invasion of Poland, other then making Stalin die, by an act of god, and having Molotov take over.