By default, all inspectors in Fiddler are read-only. To edit a Session (and have that action have any effect on what's sent or received) you must do so when the traffic is paused at a breakpoint. See
http://www.telerik.com/blogs/breakpoints-in-fiddler
If you want to edit the request or response after the session is completed (e.g. to clean it up before reissuing or saving or whatever) select the Session in the Web Sessions list and either hit F2 or right-click and choose "Unlock for editing" from the context menu.