Simplified Common Lisp reference
Symbol class: Conses, Lists and related functions
Symbol type: function
rplacdconsobject => cons
Argument description:
cons a cons cell
object an object

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)
Function indexFull documentation for rplacd (HyperSpec)