Add instructions for manual testing

docs/DeveloperGuide.md

@@ -565,15 +565,16 @@ testers are expected to do more *exploratory* testing.
    1. Other incorrect delete commands to try: `delete`, `delete x`, `...` (where x is larger than the list size)<br>
       Expected: Similar to previous.
 
-1. _{ more test cases …​ }_
-
-### Saving data
+### Undoing the last operation
+1. Undoing an execution that modifies the CampusConnect data
+   1. Prerequisites: Perform any operation that modifies the state (all executions except for list and find) to ensure there is an action to undo.
 
-1. Dealing with missing/corrupted data files
+   1. Test case: undo 
+   Expected: The last operation is undone, restoring the previous state. The list updates accordingly, and a status message confirms the undo action. The timestamp in the status bar is updated.
 
-   1. _{explain how to simulate a missing/corrupted file, and the expected behavior}_
+   1. Test case: undo immediately after starting the application (with no operations performed)
+   Expected: No undo operation is performed. An error message appears in the status message, indicating there is no action to undo. The status bar remains unchanged.
 
-1. _{ more test cases …​ }_
 
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 ## **Future features**