Though I could swear there used to be a way to do this in MS-Office, I can't find it now myself. However, one can do it in the printer settings for many printers. When I go to print an envelope, if I open Printer Properties from within the MS-Word print dialog, then go to the Page Setup dialog, there is a checkbox for Rotate 180 degrees, which does just that. You must also remember to rotate your envelope 180 degrees from normal, too, if you want the flap to be in its usual position at the top when you do this.
If you have an actual envelope document, you can specify to always use those printer settings for it so that you only have to go through this once.
Brian - Windows 10 Pro, 64-Bit, Version 1903, Build 18362
Many of the insights of the saint stem from his experience as a sinner.
~ Eric Hoffer