Poles could not hold the Germans out. Not by themselves. Frankly, Polish commanders never believed they could. Poles had a lot of (grudging) respect for German army, altough they still miscalculated numbers of German equipment - simply because they couldn't believe Hitler was ready to kill his own economy by so rapid rearmament.
Tizoc is right about organization of Polish army, but I personally would put Polish forces a little farther from the border. Polish battleplan was to fight Germans close to the border (1 phase) to show that Poland was ready to fight and ensure western Allies' help. Then Polish forces were supposed to withdraw to more defensible positions. Unfortunately, German mechanized forces moved faster wreaking chaos while Polish troops moved slowly because of Luftwaffe attacks and thousands of refugees blocking the roads. Bad communication did the rest.
It would have been also nice if western Allies didn't force Poland to call off general mobilization in August 1939 - French ambassador threatened even with stoping any French help of Poland if the war were to be started because of Polish "provocative" behaviour. That German army was already mobilized, he somehow missed.
I think a nice POD would have been elimination of Marshall Śmigły-Rydz - death in an accident or of illness, say, in 1938. Hopefully, his place would have been taken by general Kazimierz Sosnkowski. Politically he was not so ambitious as Śmigły so perhaps he would have concentrated only on military side of conflict.
Anyway, let's assume Poland has better organized and coordinated defense and Polish army is fully mobilized. What's then.
The campaign in Poland lasts longer, German losses are much bigger, but without western help the result is pretty much the same. Poland falls and is fully under German occupation.