Disconnect the power supply to the existing fixture by turning it off at the circuit breaker.
Use a step ladder to reach the existing fixture, and unscrew the light bulbs from the existing fixture.
Use a screwdriver to remove the mounting bolts that secure the fixture to the junction box.
Disconnect the wires from the junction box and mounting fixture. The wires should be connected using plastic wire caps that twist off.
Use wire strippers to cut the ends off the junction box wires, and strip 1/4 inch of insulation off the ends. Doing this will provide good contact for the wires when they are connected to the new fixture.
Use wire connectors to connect the wires from the junction box to the new motion detector light fixture. Connect the black wires together and the white wires together.
Connect the green or copper wire from the junction box to the inside of the new fixture on a screw. The screw is painted either blue or green, and it is the mounting screw for the ground wire.
Insert the wires into the junction box, and attach the new fixture to the box using the supplied mounting screws.
Screw the bulbs into the sockets of your new motion detector lights.
Set the sensitivity, and aim the motion detector according to the manufacturers instruction for your model.
Reconnect the power supply by turning the circuit breaker back on to the fixture.