To decrease speed:

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Push the SET- switch down and release it immediately. The cruising speed will decrease by 1 km/h (1 mph) each time the switch is operated in this manner.
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Push the SET- switch down and hold it while monitoring the set speed on the cluster. The cruising speed will decrease to the nearest multiple of ten (multiple of five in mph) at first, and then decrease by 10 km/h (5 mph) each time the switch is operated in this manner.
Release the switch at the speed you want to maintain.