How do you turn down the Fan Speed on a N9MP2075B12A3 Furnace?

You Can't Change the Speed.

Another Answer:That depends on the motor that is used in the furnace and whether it is belt drive or direct drive.

Many direct drive fan motors have multiple winding connections, allowing you to change the fan speed by changing the connection to the windings. To do that, refer to the diagram on the cover plate on the side of the motor. If you are AT ALL confused by the diagram or if you haven't done something like that before, contact a heating professional or electrical contractor.

Many belt drive fan motors have pulleys with multiple "steps", allowing you to change the speed that way.