If I were to change my bulb, I would simply disconnect its power, lay the unit on its side (be careful with tubing, and you might as well keep your pumps on as there will still be water in them regardless.. what's a small or large mess is still a mess), unscrew all four screws (two on top, two on bottom), pull caps off, pop the rotating connector piece off the top end and pull the bulb out. Do not touch the new bulb with anything except the little black piece that is on your old bulb. Slide the new bulb in, pop the bottom electrical connector on, screw the bottom cap on, pop the top electrical connector. Leaving the top cap off, plug the power back on, turn all your lights off, and turn the unit on. If it glows slightly, screw your cap on with it still on so you can make sure your connections stay. Then I set the unit up vertically to run properly. (In-hose on bottom, out-hose on top).
Again, this is just how mine is setup. Your's may be completely different.