Changing the Content
In Text View, DiffDog provides syntax coloring and other visual aids such as line numbering, indentation, and bookmarking, as well as powerful Find and Replace functionality that can help with editing (see Using Visual Aids in Text View). A history of changes is maintained for each file separately, and unlimited Undos are allowed.
If you have the Compare while Editing 
option activated in the Diff and Merge menu, the comparison of the files is refreshed continuously as you type.
To edit files:
•Type in your changes in the respective pane of the File Comparison window.
To undo changes made in a pane:
•Place the cursor in the pane and click Edit | Undo or press Ctrl+Z.