The simplest way is to discharge it with a resistor. The common term is "bleed resistor" because the resistor "bleeds" off the charge.

Taking into account the max voltage on the capacitor, choose a resistor with enough resistance so that the power dissipated by the resistor (v*v/r) won't exceed the resistor's power rating. For big capacitors, that is, high voltage and high capacitance, this may mean it will take a while to discharge.