Bind magit-unstage-all to 'U'.

* magit.el (magit-unstage-all): New.

magit.el

@@ -427,6 +427,7 @@ Many Magit faces inherit from this one by default."
     (define-key map (kbd "s") 'magit-stage-thing-at-point)
     (define-key map (kbd "S") 'magit-stage-all)
     (define-key map (kbd "u") 'magit-unstage-thing-at-point)
+    (define-key map (kbd "U") 'magit-unstage-all)
     (define-key map (kbd "i") 'magit-ignore-thing-at-point)
     (define-key map (kbd "?") 'magit-describe-thing-at-point)
     (define-key map (kbd ".") 'magit-mark-thing-at-point)
@@ -728,6 +729,10 @@ Please see the manual for a complete description of Magit.
   (interactive)
   (magit-run "git" "add" "-u" "."))
 
+(defun magit-unstage-all ()
+  (interactive)
+  (magit-run "git" "reset" "HEAD"))
+
 ;;; Branches
 
 (defun magit-checkout (rev)

magit.texi

@@ -130,7 +130,7 @@ into the hunk and type @kbd{s}.  Likewise, to unstage a hunk, move
 point into it and type @kbd{u}.  If point is in a diff header when you
 type @kbd{s} or @kbd{u}, all hunks belonging to that diff are moved at
 the same time.  To move all hunks of all diffs into the staging area
-in one go, type @kbd{S}.
+in one go, type @kbd{S}.  To unstage everything, type @kbd{U}.
 
 Before committing the changes in the staging area, you should write a
 short description of them.