; we define a function that returns 6 if the input x
; is 3, otherwise it returns x
(defun my-replace (x)
    (cond
      ((equal x 3) 6)
      (t x)
    )
)

; we "map" this function over a list
(mapcar #'my-replace '(1 2 3 4 5 6))

; we should get: (1 2 6 4 5 6)
(quit)