Undo
The Undo 
( Alt+Backspace, Ctrl+Z ) command enables you to undo an unlimited number of previous actions for each document separately. To undo previous actions in any document, place the cursor in that document and select the Undo command. You can switch among documents in multiple open file comparisons. In each document, the Undos will continue from the point where it was last left.
Please consider the following when using the Undo and Redo commands:
•The Undo and Redo commands also undo and redo merged differences an unlimited number of times.
•It is important to place the cursor in the document that has been changed. For example, if you merge a difference from the Left Pane to the Right Pane, then the Undo for this action will only be available if the cursor is placed in the Right Pane (because this pane contains the document that has been changed).
•The Undo and Redo history is lost when the view is switched from Text View to Grid View, or vice versa.
•The Edit menu and the Undo and Redo commands are not available in Directory Comparison windows.