Windows 10 How to sort an index created in Word 2013

[Rachael Booth]

I've just finished a history book I've been writing and created an index. However, when I add an entry to the index, i.e. "Sarah Smith", it shows up in the index just as selected - "Sarah Smith". I need the index to show last names first, then first names, then the page where the entry can be found. I physically changed the names to the format I need and then tried to sort the index but am told there are no fields (like in Excel) so it won't sort anything. Is there any way to do this that Word or any Google searches aren't telling me?