While working with digital images,you can always take back an unwanted actions or multiple actions at a time. The History panel records each change applied to an image and list each one as a separate entity in the panel, with the one in the bottom being the most recent action action.
N.B If history panel is not present on the chosen workstation and then navigate to windows menu and choose History .Moreover you can go to Edit Menu and and step Backward to undo and step forward to redo. Lastly you can also press alt ctrl z to undo and shift ctrl z to redo.