Convert simple SQL Query to WordPress query

The function you are looking for is wp_update_post().

You would want to have your posts which you want to update, then set up a loop to update each one individually.

// However you get your posts, ID's, etc.
$posts = get_posts( array( 'post_type' => 'post' ) );

for ( $posts as $post ) {
    wp_update_post( array( 'ID' => $post->ID, 'post_parent' => 8 ) );
}

For a generic SQL call, look at the $wpdb object, and it’s prepare and query functions:

global $wpdb;

$query = $wpdb->prepare( 'UPDATE wp_posts SET post_parent = %d WHERE post_type = %s', 8, 'my-post-type' );

$wpdb->query( $query );

You could also look into $wpdb->update – it may suit your needs better in the future.