Turn off the main power in the room you will be working. Do this from your home's circuit breaker.
Unscrew the canopy from the existing fixture, using a flat-head screwdriver. Remove the canopy.
Unscrew the crossbar, using a flat-head screwdriver, from the electrical box. Remove the entire lighting fixture.
Twist the wires off the old fixture to disconnect them.
Wrap a copper wire around the ground screw. Place the ground screw in the threaded hole of the electrical box. Tighten the screw with your hand.
Place your new fixture's canopy to the ceiling, making sure it's aligned with the crossbar. Adjust the length of the fixture's pipe until 3/8 inch of the threads on the screw collar loop extend through the canopy. Screw the fixture's lock nut into the crossbar and tighten with your hand.
Screw the crossbar into the electrical box, using the flat-head screwdriver.
Connect the white wire (neutral) from your new fixture to the white wire (neutral) of the electrical box.
Place all of the wires inside the electrical box. Adjust the new fixture's canopy so that it is over the threaded support. Screw the fixture's decorative nut onto the canopy to ensure that it is secure.
Screw light bulbs into the new fixture. Read the directions that came with the fixture to ensure the bulbs are the correct wattage. Turn the power back on from the circuit breaker.