To do this, you basically need to make the Polish Air Force more powerful and effective. Poland lies exposed on the flat ground of the northern European plain. It has no natural defenses which can protect ground forces from enemy air power and force enemy to have go fight in an inferior position.
Blitzkrieg worked so effectively in Poland because the Luftwaffe was able to soften up Polish defenses from the air, so by the time the Wehrmacht came rolling in, the Poles had already been devastated.
If the Poles could have prevented the Luftwaffe from establishing total command of the air, the Polish Army may have held out for significantly longer. To do this, they would need a sizable air force with well-trained pilots, and lots of anti-aircraft weaponry with highly skilled gunners.
Once the Germans began losing aircraft at an intolerable rate, they would have had to modify their tactics and start using less air power, so the fighting would largely be conducted by ground troops. Without air power to help them, German casualties would have been heavier and their advance slower. No massive carpet bombings of cities means that they will have to lay siege and fight it out street by street, and take days to secure one city.