To decrease Smart Cruise Control set speed:

Follow either of these procedures:
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Move the switch down (to SET-), and hold it. Your vehicle set speed will decrease by 5 mph (10 km/h). Release the switch at the speed you want.
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Move the switch down (to SET-), and release it immediately. The cruising speed will decrease by 1.0 mph (1.0 km/h) each time you move the switch down (to SET-) in this manner.
You can set the cruise control speed above 20 mph (30 km/h).