Removing Author Name from Comments Already in Word:
Word for Windows 2013/2016/Office 365
Open the document that has been saved with comments that you want to make anonymous.
For safety, save a copy of the document under a different file name.
From the menu options at the top of the screen, click "FILE".
In the screen that opens, click on the "Check for Issues" box; a drop-down menu will appear — from that, choose "Inspect Document"
In the "Document Inspector" window, check ONLY the "Document Properties and Personal Information" box. All other boxes should be unchecked. MOST IMPORTANTLY, do not check the "Comments, Revisions, Version, and Annotations box" — checking that box can lead to the deletion of all comments in the document.
After confirming that only the "Document Properties and Personal Information" box is checked, click the "Inspect" button at the bottom of the window. When the next screen appears, click on “Remove All” (to the right of “Document Properties and Personal Information”). After the inspection is done, click the "Close" button at the bottom of the window, then hit the Esc key to return to your document.
Finally, use "Save As" (File -> Save As) to save the file with a new name. Wait a few seconds, and the comments in the file with the new name will become anonymous.