It's super easy... turn the power supply off. Unscrew the 1 screw ontop of the light fixture. Take it out.. it has lots of slack on the other side to pull out of the pool to work on. Unscrew the many screws on the back of light and take the fixture out of the molding. Break the seal between the rubber and the glass (remember how the seal is on it). Replace the lightbulb with same voltage lightbulb (mine was 12volts and cost $15.00 at the pool store). Put it back together... I put a nice bead of silicone around the rubber seal before attaching it back onto the glass.
It's that easy!