If you want to discard all the changes you have made to the current VC fileset, type @kbd{C-x v u} (@code{vc-revert}).  This shows you a diff between the work file(s) and the revision from which you started editing, and asks for confirmation for discarding the changes.  If you agree, the fileset is reverted.  If you don't want @kbd{C-x v u} to show a diff, set the variable @code{vc-revert-show-diff} to @code{nil} (you can still view the diff directly with @kbd{C-x v =}; @pxref{Old Revisions}).  <del>Note that @kbd{C-x v u} cannot be reversed with the usual undo commands (@pxref{Undo}), so use it with care.</del>