Apple ID/iCloud popup

I had this issue too, as well as not finding the right answers. Then I finally figured out that you don't actually have to uninstall it. Just go to Settings > Apps > then scroll down in Apps & features and select iCloud and click advanced options. Scroll all the way down to reset (above uninstall). Resetting from here doesn't delete the data in icloud, just the preferences and sign in credentials on the computer. That's literally all I had to do. Don't even have to mess with reinstalling. Just reset and reopen icloud to log in lol.

I finally got it to work. I deleted the iCloud app from my Windows computer, reinstalled it, and I was able to use the correct apple ID. Hope this works for you.

I've changed my email address for iCloud,

Then see this Apple article: What to do after you change your Apple ID or password

What to do after you change your Apple ID or password - Apple Support

I have this same exact problem. I have no problem with icloud on my phone or ipad, but my Windows PC is another matter. My ID is greyed out, and I cannot change it.