RPLACD function changes CDR part of CONS cell to specified value. See RPLACA, SETF, CONS.

This can be also writen as (SETF (CDR cons) object).

(let ((my-list (list 5 3 6 2))) (rplaca (cdr my-list) '(x y)) my-list) => (5 (X Y) 6 2)