Replacing a ceiling fixture is simple. The old fixture's hardware often can be reused, and the new fixture usually comes with necessary hardware too.
Compare the wattage of the new fixture with the old one to be sure it provides the amount of light you want. Choose a fixture with a canopy large enough to cover any imperfections in the ceiling or use a medallion.
Wires in a ceiling box may contain cracked or brittle insulation due to overheating. Repair these before installing a new fixture. Ground the new fixture if possible. If it is being installed in a metal box, connect the fixture ground lead to the box and to the house ground wire. Check local building codes.
About 30 minutes to remove a fixture and install a replacement, as long as there are no problems with the hardware
Screwdriver, strippers, side cutters, voltage tester, ladder
Stripping wire, splicing stranded wire to solid wire
Spread a drop cloth on the floor below; set up a stable, nonconductive ladder
Materials Replacement fixture, wire nuts (the ones that come with the fixture may be too small), electrician's tape
Fold the wires up into the box. Start one mounting screw, then the other, then tighten them. If the fixture has keyhole-shape screw holes, attach the screws to the box; slip the fixture over the large holes. Rotate the canopy so the screws fit into the smaller slots and tighten the screws.
Some fixtures mount with a nipple (short threaded pipe) and a nut in the center rather than two screws. Install a strap and screw in a nipple. If the nipple that comes with the fixture is too short or too long, purchase another one.
If the new canopy is not large enough to cover holes or unpainted portions of the ceiling or if you simply want to add a decorative touch, purchase a medallion. Hold it against the ceiling while you wire the fixture. Before tightening the canopy see that the medallion is centered.