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Turn off the power to the light fixture at the circuit-breaker box.
Remove the screws holding the fixture in place. Gently pull the fixture out of the junction box, and test it with the circuit tester to be sure the power is off. Disconnect the wires and remove the existing fixture.
Note: Before installing a ceiling fan in an existing light fixture, make sure the existing junction box is braced securely. A ceiling fan is much heavier than a typical light fixture and could cause some junction boxes to break through the ceiling.
If there's attic access directly above the fixture, go into the attic and remove the junction box from the ceiling joist (Image 1). Don't worry about damaging the junction box, but be careful not to break through the ceiling. If attic access is not available, break the junction box free and remove it from below.
If there's attic access, use a wood block fitted between the ceiling joists (Image 2). Slide a heavy-duty junction box over the wood block (Image 3) so it is positioned over the hole in the ceiling, and secure it with 1-5/8" all-purpose screws. Drive sixteenpenny nails through the joists into the ends of the block to secure it in place.
Install a brace to support the ceiling fan.
A drywall-mounted ceiling-fan support can be used if there isn't access to the area above the ceiling. Slide the support into the hole for the junction box, and adjust the clamps on the support to secure it to the joists in the ceiling.
Install the ceiling fan as directed by the manufacturer's instructions.