So you can replace just the top capacitor with a 4 uF capacitor. The higher the capacitance, the faster the speed will be. The original 2.5 uF capacitor on the bottom will connect in parallel with the top capacitor you change to 4 uF, giving a total of 6.5 uF when the module is set to medium. This should be a good combination, since most of the fan run capacitors I found online were between 3 to 6 uF.
My father also suggested, since these are probably not well made PCB's, to use de-soldering braid to remove the solder from the pins, and make sure your iron is hot. Don't spend too much time on any pin or you will delaminate the circuit from the board. Using a strong desoldering tool on cheap PCB's will almost certainly delaminate the circuit.
