Thanks for your answer, Christopher, and for replying so quickly. I understand now how to work with an alternate version of my theme.
As it turned out, I didn’t have to use this technique since I discovered how to recover what I thought I had deleted. By deleting the menu items I mentioned, I thought I had deleted the pages they linked to as well. Not the case. I found them by going to Dashboard/Pages and found them in the list. Restoring the original menu items was as simple as going to Customize/Menus/Add a Menu, typing in the menu name, clicking “Create Menu” then “Add Items” and selecting the appropriate page from the “Pages” list.