Voltage ratings on capacitors give the lowest voltage that may destroy the capacitor. This means that the capacitor is permanently destroyed as a capacitor, even if the voltage is removed. It may test as a short circuit, or it may break down at a lower voltage next time the capacitor is used.
Air spaced capacitors are usually not destroyed by high voltage but will arc over if the voltage is high enough. Removal of the voltage is sufficient to restore the capacitor to full capability.
A 5.5 volt capacitor would be unusual. The DTR line from a computer's serial port may carry up to +15 Volts or -15 volts, so the capacitor should be rated at at least 25 volts. This depends on the circuit, of course, and it may already have measures to make a low voltage capacitor safe.