;;replace second param with first param
(defun replace (new old l)
  (cond
   ( (null l) l)
   ( (eq old (car l)) (cons new( cdr l)))
   ( t ( (cons (car l) ( replace new old (cdr l)))))
   )
  )
;;demo replace
(replace 3 6 '(6 5 7))
;(3 5 7)

;;recursion replace all second param with first param
(defun replaceall (new old l)
  (cond
   ( (null l) l)
   ( (eq old (car l)) (cons new (replaceall new old (cdr l))))
   ( t (cons (car l) (replaceall new old (cdr l))))
  )
)
;;demo replaceall
(replaceall 3 6 '(6 5 7 6))
;(3 5 7 3)