there have been a few inquiries lately on how to adjust this and I've thought to post this answer as I have used it a few times.  

I suggest you have a look at the service manual section 6-63 (pp. 1K4) for the adjustment procedure.  It states:  "When properly adjusted the landing gear warning horn should operate when the power is reduced below 10 to 12 inches of manifold pressure with the airplane in normal descent and the landing gear retracted."

There are a few different procedures there depending on serial number.  But in essence it involves running the aircraft up and setting the manifold pressure to 4 inches above the desired warning pressure.  So if you wanted it to trip at 12 inches you would set the manifold pressure to 16 inches.  Mark the throttle with something that is difficult to dislodge - perhaps with masking tape.

You put the aircraft on jacks, retract the gear, set the throttle to the marked position, climb into the requisite contortionist position under the panel, loosen the switch securing nuts, and slide the switch until it just starts to sound.  Secure the switch and remove the tape.

Presumably it could be adjusted a bit, and then checked with a flight test, but I am going to recommend that you follow the service manual. 

Attached file has a picture of the switch behind the panel.