Changing the HID ballast in a Prius is not as problematic as changing the HID bulbs themselves. The ballast, after all, is a box that serves as a regulator to electrical signals that reach the light. Ballasts are also precisely matched to the HID bulbs they power, so any replacement of a ballast should match the voltage and wattage outputs of the old ballast precisely. Ballasts in general are part of any HID-light installation kit.
Disconnect the power cable from the car battery.
Detach and remove the old Prius HID ballast. There is one for each headlight.
Connect the wires from the ballast to the HID lights, and connect the other wire harness to the Prius' battery. The plugs are different: bulb connections will not connect to battery connectors.
Install the new Prius HID ballast in the same space as the old ballast(s).
Reconnect the battery cable and test the HID headlights.
