1993 Ford Aerostar need to know how to turn the idler down

Since your question is old, you may have found the answer. The manual I have says that the idle air control valve controls the idle speed, and no adjustment is possible. Get a repair manual from the library, to find out how to test it. The part is about $65.00 new, or less from a salvage yard. You'll probably have to show them where it's located, and what it looks like. Good luck.