Supposing that you can't kill of Hitler, Himmler, Georing and Goebbels and September 1939 is inevitable, I'd try and give Germany a very bloody nose in 1939 and stop the whole wrotten 6 years from happening.
1. Push the Polish govenment in 1935 to begin re-equipping their airforce and army. Poland had some good equipment in 1939, it just didn't have enough of it.
1a. Give the airforce the P.24s in 1936-1937 to replace the P.11c and then introduce the P.50 in 1937-1938. Sure, its not much, but look what the Polish airforce did with a handful of superbly trained and dedicated pilots in the P.11c. Replace the PZL P.23 'Karas' with the uprated P.43 with the Bristol Mercury. Make sure the P. 37 'Los' is available in greater numbers as well.
1b. Make sure that production of the 7TP is increased and troops properly trained in their use. The 7TP was far better than the early Panzer I and II and a match for the Panzer III. The 37mm was a good gun. Produce the TKD 40mm armed tank destroyer and the 20mm armed version of the TKS in better numbers as well.
2. Give Poland 12 months notice of German invasion plans. Allow them full mobilisation and readiness for the German attack.