There is no such switch, as far as I know.

The various things that make it run better when cold:
1) The cold start switch - not really related to cold running, just cold starting. It only activates when the starter is cranking and the engine is cold. If you have a really hard time starting when cold, this could be the problem.

2) Idle air bypass valve - this bumps up the idle speed by allowing more air to sneak around the throttle valve. It is the aluminum doohickey on the side of the head, connected to the intake manifold by a couple of fairly sizeable rubber hoses (3/4 inch diameter?). These are very commonly stuck, either shut or open. If open, you'll have a persistent fast idle when warm, and many people just plug the hose. If shut, it will idle too slowly when cold, and you'll need to keep your foot on the pedal until warm.

3) ECT - engine coolant temp sensor. On the front of the head, near the idle air valve. A small electronic sensor that tells the computer when the engine is cold. This causes the computer to pretty significantly richen the mixture. Bad sensors, bad wiring between the sensor to the computer can cause the computer to think it is cold all the time (and always run rich) or always hot (and run lean when cold).

4) IAT - Intake air temp - although frankly this has a very modest affect on the mixture, and really only changes when the weather is cold or hot, not related to engine temp.
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I'm JohnMc, and I approved this message.