stm32f4 discovery user button debounce
I believe C38 is not fitted because this circuit does not debounce the way that ST intended, I did some spice simulations just to check (I would suggest that you could also do this for the exercise). I would move C38 in parallel with R39. Then tweak the values of R38 and R39 (I'm assuming you're familiar with time constants of RC circuits) to get the desired debounce.