Haven't made it to Lynda yet, but I did some reading and came up with the following solution.
1. Turned my .epub into a .zip file by changing the file extension. Did not extract files.
2. Dragged and dropped only toc.ncx and content.opf onto my Desktop.
3. Edited the code in each file using Notepad++ (easier to keep all that code straight than in regular notepad) and hit Save.
4. Dragged and dropped the edited files back into the (still zipped) .zip file.
5. Changed the file extension from .zip back to .epub
6. Ran through ePub Validator. Success!
