No capacitor does not increase the voltage. but they can be used in many circuits that create output voltages higher than those of the input. Capacitors are energy storage devices.

What do capacitors do to voltage?

A capacitor opposes changes in voltage. If you increase the voltage across a capacitor, it responds by drawing current as it charges. In doing so, it will tend to drag down the supply voltage, back towards what it was previously. That's assuming that your voltage source has a non-zero internal resistance.

Do capacitors reduce voltage?

Capacitors work their magic by storing energy.

The strong attraction of the charges across the very short distance separating them makes a tank of energy. Capacitors oppose changes in voltage. It takes time to fill up the plates with charge, and once charged, it takes time to discharge the voltage.