Ceiling fans are heavy and they vibrate. A fan installed on a standard ceiling fixture box could come loose and perhaps crash to the floor.
The first step in adding a ceiling fan is to inspect the existing electrical box. Shut off power to the circuit. Test to confirm that the power is off and remove the existing ceiling fixture.
Most building codes require ceiling fans to be mounted on special fan-rated boxes, which are made of metal or strong plastic and have deep-threaded holes for the mounting screws. The box must be mounted firmly, either by attaching it directly to a framing member or by using a fan-rated brace.
Replacing a ceiling box is a messy job. Work carefully to avoid cutting through wires hidden in the ceiling.
About 2 hours to remove an old box and install a new one
Ladder, screwdriver, hammer, drill, pry bar, utility knife, flashlight, wrench or groove-joint pliers, perhaps a reciprocating saw
Cutting drywall, prying boxes out without damaging surrounding ceiling or wall
Spread a drop cloth on the floor below; set up a ladder.
Fan-rated box, box brace