You first need to change the "Preferences" for Acrobat/Reader to not use Log-in-Name as author name, uncheck the option "Always use the Log-in-Name as author name.". Then select a comment/annotation, right mouse click, select the "Properties" context menu option, change the author name on the "General" tab, and click the "OK" button. The name for that comment has now been changed.
To make this default comment style, select a comment of the type you want to make the default for with all the properties you want as default, right mouse click and select "Make current properties default" from the context menu.