step down

verb

1. Used other than figuratively or idiomatically: see step, down.

   “to step down carefully from a train”

2. To resign or retire from an office or position of authority.

   “She now wants to be a Paris MP. But Fillon wants the same Paris seat when he steps down, after next year's election. Both have their eye on the main prize: running for Paris mayor in 2014.”

   “Late on the previous Friday (September 2), AWC had announced that Managing Director Phil Whittingham "has decided to step down with effect from September 15".”

3. To reduce gradually or incrementally.