Estimate the height of your cathedral ceiling if you do not already know it. If you are estimating, allow 10 feet per story. Rent a scaffold kit or electric lift that will allow you to reach that height, keeping in mind that the top platform height should be less than the ceiling height to allow you to stand upright while working on the fixture. Subtract your own height from the ceiling height to get the best estimate of how high the platform can be.
Set up your lift or scaffold kit directly under the light fixture you want to change. Follow all safety instructions that were included with your rental, including all railings and crossbars on scaffolds or all legs or outriggers on lifts.
Shut down the circuit breaker to the light fixture you will be changing.
Load your new light fixture, flat and Phillips head screwdrivers, wire strippers and wire nuts into the basket of your lift. If you are using a scaffold, place these items in a 5-gallon bucket and leave it near the scaffold's base.
Get into the lift and raise yourself to a height that is comfortable for you to work on the fixture. If using scaffolding, use a heavy cotton rope that is at least 6 feet longer than the height of the top rail on your scaffold and loop it around your arm. Climb to the top of the scaffold with the rope. Tie one end of the rope around the top rail of the scaffold once you are on the top platform, then lower the other end down to the ground and have a helper tie it securely to the bucket's handle. Pull the bucket with your supplies up to you and set it on the platform.
Remove all screws that hold the current light fixture in place on the ceiling and pull the light fixture away from the junction box it is attached to. Turn the wire nuts that tie it into the building's wiring counter-clockwise to remove them. Set the old light fixture in the base of the lift or on the scaffold platform, and be very careful not to bump into it while installing the new fixture.
Install your new light fixture according to its instructions, then pack all tools and the old fixture back in the bucket and lower it to the ground, or set them all in the base of the lift, and lower yourself to the ground.
Keep the lift or scaffold in place until you have tested the fixture to ensure it works.