[Newish user]
I transferred some .XLS files from my PC to the Cloud Drive website.
Sure enough, I could view them and edit them on my Kindle Fire HD.
But I don't see how to upload the edited files to the Cloud Drive website or to my PC's Cloud Drive folder where they'll be accessible to my PC.
If I tap Sync on the Quick Settings menu, the Sync icon rotates through 360 degrees, but as far as I can tell nothing else happens.