Microsoft Word has several options for creating and modifying numbered lists, but reverse numbered (or countdown) lists aren’t one of them. I’ve never had to use one in my technical documentation, but they’re common enough, especially at this time of year when people present best of the year lists.
Of course, you can always manually number the items, but if you want automatic numbering things get a little more complex. The WordTips Newsletter presents a couple of different options; one being a fairly simple compound field, the other a macros.
BTW, if you don’t know about the WordTips newsletters, you should check them out. You won’t be wasting your time.