How do I switch a 220 air compressor to 110? What if the diagram is not underneath?

If it is a straight 220VAC compressor, you can't run it on 120. If it is a 120 or 240, look under the wire connection cover, most diagrams are on that. Usually there are 4 posts. With 240 you use 2 hots and a ground, no neutral. With 120 you would use 1 hot, 1 neutral and 1 ground, leaving the second hot terminal empty.