Maybe that old sconce or chandelier is broken beyond repair, or perhaps you're just tired of its leftover-from-the-last-century look. You can usually replace faulty sockets or wiring in light fixtures (see How to Fix a Lamp), but if this isn't possible or you want to make a change, replace the entire fixture.
Start by turning off the power to the fixture's circuit at the service panel.
When you remove the fixture base on a wall or ceiling-mounted fixture, or the cover plate on a strip fluorescent fixture, test the wiring to verify that the power is indeed off. Probe each set of insulated wires with one lead of a neon tester and the metal box or grounding wire with the other. If the tester lights, the power is still on.
Remove the screws or cap nuts that secure a globe or glass light shade.
Remove the screws or cap nuts that secure the fixture, fixture body or canopy to the outlet box. Support the full weight of a heavy fixture during this step and the next. Get help for a heavy one, especially if you are working overhead from a ladder.
With the wiring in the outlet box now accessible, repeat the power test described in step 2. When you've confirmed that there's no power in the outlet box, remove the wire connectors that join the fixture wires to the house. If the fixture body is grounded, remove the screw or nut that secures the grounding (green) wire.
You must secure fixtures that weigh over 50 lbs. (23 kg) independently of the electrical box. If you have access from above, install wood bridging between the joists above the box. Then screw the box into the bridging from below. If you don't have access to install a support from above, purchase retrofit mounting hardware for a ceiling fan and install it according to the manufacturer's instructions.
Compare new mounting-hole requirements with the existing provisions at the outlet box. If necessary, purchase a new mounting strap or a universal mounting plate, which will accommodate virtually any fixture.
Connect the new fixture to the same wires that connected the old fixture, and mount the fixture to the box with screws or cap nuts as required. Create a loop in the wires with long-nose pliers, then wrap them clockwise under a terminal screw. Or use electrician's pliers to twist the wires from a prewired fixture together with the house wires, then secure them with wire nuts. Cut off excess with wire cutters.