How to modify "Classic" style for Word 2013 indexes

To add the group headings to the "from template" index, after you've inserted the index, right-click anywhere inside the index and click Toggle Field Codes. In the code, after the keyword INDEX, add the switch

   \h "A"

to make it look like

   { INDEX \h "A" \c "2" \z "1033" }

(some of the other switches may be different). Then right-click again and click Update Field.

If you would rather start with the Classic style and change the formatting, here's how:

The only thing the Modify button in the Index dialog lets you do is to modify the nine paragraph styles named Index 1, Index 2, ... Index 9. Although that button is disabled when you choose any of the built-in indexes (and no, I don't understand why the program does that!), you can use the standard style-modification methods outside of that dialog.

Because the Index N styles aren't ordinarily included in the Quick Styles gallery on the Home ribbon, you'll need to use one of the other ways to get to the Modify Style dialog. The simplest is to press Ctrl+Shift+S to open the Apply Style box, type Index 1 in the Style Name box, and click the Modify button.

In the Modify Style dialog, click the "New documents based on this template" if you want to change the default for all future documents based on the current template. Then change the font name and size. Click the Format button at the bottom left, click Paragraph, and change the line spacing. Click OK.

Repeat for the Index 2 style that's applied to second-level subentries. Repeat for Index 3 through 9 if you use those levels. You may also want to modify the Index Heading style, which formats the letter at the start of each group.