Firstly, ease up on the secrecy and train up troops to use their (pretty decent) AT rifles and issue them promptly. They would still have lost, but exacted a much higher price on the most important arm of the German army and maybe added another week of defiance. That's an easy win as they had the rifles in sufficient numbers but needed proper training and distribution.
Second, update a few more of the tankettes with 20mm. Again, even another dozen or two would have slowed and disrupted attacks. There were plans to do more conversions as I understand it [1] but not soon enough.
Tanks. Nice, but in 1939, more tankettes might have been a better option. They were low profile, easy to conceal and nasty against infantry znd softskin units. On the defensive, they would be useful for spoiling attacks and generally disrupting unarmoured units.
Fighters needed to be updated sooner. Though agile, they needed something more modern, faster and with more guns. This is the hardest as it needs lots of money and retraining.
Better defensive planning and mobilisation. Even a modest improvement in initial deployment may have helped. Again not a big resource sink, but asking 'we could be attacked from all sides. Is a long thin crust the best option?' one more time might have led to a modest improvement in the price paid by the Germans, and particularly by their most important mobile units.
So tgat's my good ideas for today. Apart from the fighters only modest changes needed, and while not enough to win, slowing thd Germans by a week or two has big downstream impacts [2]
[1] I can't read Polish so this may not be correct, but it's a relatively easy change.
[2] A hundred or so fewer tanks and tank crews makes France just that bit slower and more expensive, and then Barbarossa or Greece is missing a few more hundred tanks and crews and a few hundred capured tanks that ITTL were destroyed in combat. Plus fewer captured vehicles, possibly less pressure on the BEF so personal weapons, bren guns and AT rifles and a few guns are evacuated with the troops, which maye leads to 6 pounder production starting earlier. So a big deal eventually.