I just (about 4 days ago) did a Windows reinstall from the web. I found I could not change the default pic view from Microsoft Photo to irfanview V4.62 64bit. After attempting the fixes found here, I found the "repair" and "reset" buttons in the SETTINGS / APPS / DEFAULT APPS / PHOTOS section. I first did the repair, then the reset. Next I went to the image files I wanted to associate with irfanview V4.62 64bit clicked on those images (ie... jpg) and was able to see the "set as default" button on the bottom right of the window. Now my associations are back to where I want them.
Thank you "EVERYONE" for your posts.
Happy, Happy. Happy.