The History panel (Window > History) shows a list of the steps you’ve performed in the active document since you created or opened that document (but not steps you’ve performed in other frames, in other Document windows, or in the Site panel). Use the History panel to undo multiple steps at once and to automate tasks.

The slider, or thumb, in the History panel initially points to the last step that you performed.
Undo the last step
Do one of the following:Select Edit > Undo.
Drag the History panel slider up one step in the list.
Undo multiple steps at once
Drag the slider to point to any
step, or click to the left of a step along the path of the slider.
The slider scrolls automatically to that step, undoing steps as it scrolls.
Set the number of steps that the History panel retains and showsThe default number of steps is sufficient for most users’ needs. The higher the number, the more memory the History panel requires, which can affect performance and slow your computer significantly.
Erase the history list for the current document: