May I ask why you want to change the speed?
Do you want to change the speed for your heating, or cooling?
I want to change the speed for cooling. The blower is really fast and so loud that I cant hear my tv when the blower is on.
The cooling should be set on high speed, and the heating should be wired for medium, or low.
Would you say that your heating moves more air, and is set on high?
Usually the motor is already wired for the lowest speed.
You can verify this by looking at the diagram listed on the inside of the panel on your furnace.
If the diagram is not on the panel, you can remove the blower motor and look at the diagram listed on the motor itself.