This is a known issue. See the following WD Suupport link for more information.
The easy workaround to this issue is to use the WD My Cloud Desktop application to open the file into their Microsoft Office application, then edit, then save any Microsoft Office files that are already stored on the WD My Cloud. One obviously will have to copy the Microsoft Office files they want to work on to their WD My Cloud first.
The following link, while for the WD My Book Live and from 2012, may (or may not) also work on the WD My Cloud but I have not tried it. It involves using SSH to setup the same username/password one uses on their Windows PC. I typically copy the content to the WD My Cloud then use the WD My Cloud Desktop app remotely to open, edit and save the document to the My Cloud drive.
