Every change in OBO-Edit can be undone, and if you accidentally undo a change, it can always be redone.
To undo a change, hit Command+Z or select Undo from the edit menu:

To redo a change, use Shift+Command+Z or select Redo from the edit menu:

For information about how OBO-Edit implements undo/redo, see How OBO-Edit Understands Histories.
Note that you can't Undo changes made in the Text Editor until you've committed them by clicking the Commit button (if you are in manual commit mode) or by selecting a different ontology term (if you are in auto-commit mode). For more information about text editing, see the Text Editing help page.