Every change in OBO-Edit can be undone, and if you accidentally undo a change, it can always be redone.
To undo a change, select Undo from the edit menu:
The Undo menu item will usually contain a brief description of the operation to be undone.
To redo a change, select Redo from the edit menu:
The Undo menu item will usually contain a brief description of the operation to be redone.
For information about how OBO-Edit implements undo/redo, see How OBO-Edit Understands Histories.