I have a lennox central air m\n gcs 20--- had a new motor installed about a two years ago
The fan motor is set on high,with wires do I need to change to get a med-speed.
Thank you.

There are about 15 different motor manufactures for furnaces/HVAC units. Usually the wiring colors for speeds will be the same but would not chance it. There is a wiring diagram on Every motor printed on the side. You might have to pull the motor to see the diagram unless you have a exact model and brand name so I can look it up. Sometimes they will put the numbers on the bill for service you had 2 years ago. Use bottom for PSC motors

Normal standard colors are in the file below

Just curious, why do you want to reduce your motor speed?