step down
step down {v.}
1. To come down in one move from a higher position to a lower.
As soon as the train stopped, the conductor stepped down to help the passengers off.
2. To make go slower little by little.
The train was approaching the station, so the engineer stepped it down.
3. To leave a job as an official or some other important position.
When the judge became ill, he had to step down.
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Source: A Dictionary of American Idioms