Juan Jose

                  <?php
/**
 * Adds previous/next links to post edition window
 *
 * @return string
 */
function add_navigation_edit_posts() {
        if ( isset( $_GET['post'] ) and isset( $_GET['action'] ) and $_GET['action'] == 'edit' ) {
                global $post;

                $args = array(
                        'public'   => true,
                        '_builtin' => false,
                );
                $output   = 'names'; // names or objects, note names is the default
                $operator = 'and'; // 'and' or 'or'

                $post_types = get_post_types( $args, $output, $operator );

                $post_types['post'] = 'post';
                $post_types['page'] = 'page';

                if ( is_object( $post ) and in_array( $post->post_type, $post_types ) ) {
                        foreach ( array( true, false ) as $prev ) {
                                $p = get_adjacent_post( false, '', $prev );

                                if ( ! empty( $p ) ) {
                                        echo '<script>
                                        jQuery(function($) {
                                                $(".wrap h2" )
                                                .append('<a class="add-new-h2" href="' . admin_url( 'post.php?action=edit&post=' . $p->ID ) . '" title="' . __( 'Edit' ) . ' ' . apply_filters( 'the_title', $p->post_title ) . '">' . ( $prev ? '« ' : '' ) . apply_filters( 'the_title', $p->post_title ) . ( ! $prev ? ' »' : '' ) . '</a>' );
                                        });
                                        </script>';
                                }
                        }
                }
        }
}

add_action( 'admin_head', 'add_navigation_edit_posts' );
?>