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.
Select Edit > Undo.
Drag the History panel slider up one step in the list.
The slider scrolls automatically to that step, undoing steps as it scrolls.
The 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.