Turn off and unplug the fan.
Locate the screws on the fan that secure the housing. The housing is the cover of the fan. The screws should be on the back and under side of the fan.
Remove the screws using a screw driver.
Locate the fan motor. This will be the large component under the housing. It will have several wires connected to it as well as a bar out to the front of the fan that connects to the fan blades.
Remove the fan motor. Disconnect the wires from the motor. Some may be clipped on, others may need to be cut. Remember what wires go to what. These wires will be connected to the fan's switch. The power wire will look much like any power cord and will be the thickest wire on the fan. Unscrew the motor from the housing. Remove it. The process will vary with fan models.
Install the new fan motor. Make sure the motor fits into the fan's housing. Usually, a more powerful motor from the same manufacturer of fans will be easier to install due to similar design properties.
Connect the fan switch wire to the new motor. There should be two or three wires and terminals. A power and ground and/or variable control wire. The install manual for the motor will indicate which wires go where.
Mount the fan motor to the housing. Attach it to casing of the fan.
Reattach the housing.
Plug in the power and test out the fan.