(defun test (pos val)
  (let ((var '(a b c d e)))
    (setf (elt var pos) val)
    var))

CL-USER> (test 0 1)
(1 B C D E)
CL-USER> (test 1 2)
(1 2 C D E)
CL-USER> (test 0 3)
(3 2 C D E)