Editors can currently en/disable whether an author can see a Reviewer's uploaded review file (under Review, at the bottom of Reviewer X's section beside "Uploaded files"), but can't overwrite that file. An Editor should be able to overwrite this file, for example if the content is good but it needs to have identifying metadata stripped. 

Current workaround: download the file, and rather than allow the author to view it by checking the 'enable' box, upload a stripped version as an email attachment to the Author Notification email. 

(Courtesy of http://pkp.sfu.ca/support/forum/viewtopic.php?f=2&t=4793&p=18669#p18669)