By selecting Undo you can undo the last action you performed in the drawing area. For example, if you move a component by mistake, undo until the component is back in its original position. You cannot go back infinitely, but you can undo a chain of several commands.
By choosing Redo you can re-apply an action that you undid earlier.
Ctrl + Z = Undo
Ctrl + Y = Redo
There are a few ways you can Undo or Redo:
At the top left of CET above the Component Tabs area, click the red arrow button to Undo, or the Green arrow to Redo.
Open the Edit menu and click Undo or Redo the necessary number of times.
In the File menu, you have the option to Revert the drawing. This will undo all changes that have been made since the drawing was manually saved.