There is small screw-in bulb inside the blue lens cover that can be replaced. The bulb number is #335, 28 volts.
As I recall, if the chassis is removed and the top cover plate is taken off the socket can be unscrewed from the rear. Or if you can find a way to carefully remove the front blue lens cover, the bulb can be unscrewed from the socket by using a piece of heat shrink or rubber tubing.