There's a screw on the bottom. Undo it, and pull from the bottom, then lift off the lens. The bulb is in the back. pull the connector, and there is a spring clip of some sort that holds the bulb. Pop it off, and swap the bulbs, without touching the glass of the bulb. I would clean it with rubbing alcohol or acetone before putting it in.
You should add relays (and a high wattage bulb if desired) while you are in there. Not a big deal to do.