To discharge a capacitor, you need to de-power the device to which it is attached and, if possible, remove the capacitor. If not, you can do it in place. Use a resistor, a light bulb or a voltmeter to complete the circuit between the two leads. This will discharge the capacitor. Have someone show you how to do this in person before attempting to discharge a capacitor that holds a great deal of energy; they can do real harm.