Can you change a 35mF 425volts capacitor with a 20mf 410 volts capacitor?

Answer 1

Only if you are sure the voltage is less than 410 volts peak or it might fail. and- only if you are sure the reduced capacitance will not affect circuit operation.

For most purposes, no, you should not do that.

Answer 2

The critical thing here, is its voltage rating, which must exceed the voltage applied across the capacitor. Bear in mind that a.c. voltages are expressed in root-mean-square values (rms), so you will need to divide the a.c. voltage by 0.707 to determine the peak value of voltage, as it is this value of voltage, not its rms value, which must be exceeded by the capacitor's rated voltage. In your example, any voltage in excess of 440 V may damage the capacitor's dielectric.