Turn off the power to the light fixture's circuit at the circuit breaker panel.
Loosen and remove the thumbscrews holding the light fixture's base onto the mounting plate installed on the light fixture box. Use pliers to turn the screws if they're to tight to remove with your fingers.
Lower the light fixture off the ceiling. Loosen the wire nuts connecting the home wiring to the light fixture wiring. Cut the wires if they're not connected with wire nuts. Remove the light fixture.
Loosen and remove the screws holding the light fixture's mounting plate onto the light fixture box.
Loosen the screw or screws attaching the light fixture box onto the ceiling joist. If the box has been nailed to the joist, pry the nails out with the tip of pry bar.
Lower the light fixture box out of the ceiling.