update a posts of other custom post type

You need to use save_post action along with wp_insert_post() function for this. Something like following may work.

function tnc_update_car_details( $post_id, $post, $update ) {

    // If this is a revision, don't continue
    if ( wp_is_post_revision( $post_id ) )
        return;

    $post_type = get_post_type($post_id);

    // If this is not an invoice post type, don't continue
    if($post_type !== 'invoice')
        return;

    $car_id = '1';

    wp_insert_post(
        array(
            'ID' => $car_id,
            'meta_input' => array(
                'km_travelled' => '300',
            ),
        ),
    );
}
add_action( 'save_post', 'tnc_update_car_details', 10, 3 );

I’ve not tested though. Please let me know the result after trying.