The requirements state that you must implement the DBMain interface, so I would recommend that you implement the delete, create methods as the interface could be used by other applications.
I believe some people have thrown a Runtime Exception stating that the method is unsupported, however I don't know if they were marked down for this approach, and for the sake of a few lines I think it's better to be safe than sorry!
The assignment does state that the user interface should be designed with the expectation of future functionality enchancements.
I wouldn't recommend modifying the DBAccess interface.
If you need to add additional methods (like getMetaData or additional search methods) I would create a new interface i.e EnhancedDBAccess which includes the new methods, then in you Data class you can implement both the DBAccess and EnhancedDBAccess. Thereby not modifying the existing interface.