Well, you are not really "editing" that field, but you can control what is displayed as the "description".
The instruction for doing that can be found in the apache documentation for the "mod_atuoindex" module:
You will probably want to read the rest of that page also, just so you understand what it is doing and how it works.
There are also many scripts around that are designed to help you manage index displays, and you may find dome of those easier to use than working directly with the .htaccess file. (just search any popular script archive, such as "hotscripts.com").