Carolyn, I presume you're referring to the Desktop in Windows Explorer or File Explorer (I can't remember what version of Windows you're on). I'll also presume you're talking about Alphabetical order from A to Z with all folders appearing before the files (although that won't matter).
When you are in File or Windows explorer, navigate to the Name Split button, which acts as the Ascending/Descending toggle if you're trying to sort by name. Hitting spacebar when resting on that button will toggle an ascending sort to descending sort or vice versa. If you wish to sort on date, file type, etc., you can arrow right once you're sitting on the Name Split button to get to the column on which you wish to sort. Any of them act as the same sort of toggle as Name did, but will focus sort on that property instead of name. Brian
