Click the name of the state in the Undo History palette.

Drag the slider at the left of the state up or down to a different state in the Undo History palette.

Click the Undo  or Redo  buttons on the shortcuts bar.

Choose Undo or Redo from the Undo History palette menu or the Edit menu.

To set the keyboard command for Step Forward and Step Backward, choose Edit > Preferences > General, and choose from the Step Back/Fwd menu.