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

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

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

(let ((my-list (list 5 3 6 2))) (rplaca my-list 'bla) my-list) => (BLA 3 6 2)
Function indexFull documentation for rplaca (HyperSpec)