Web Service

To add a new comment to existing order and change the order status you can use this endpoint

/rest/V1/orders/{order_id}/comments

and the json data is

{
    "statusHistory": {
        "comment": "Your Comment Here",
        "created_at": "2018-06-05  15:22:04",
        "parent_id": {order_id},
        "is_customer_notified": 0,
        "is_visible_on_front": 1,
        "status": "pending"
    }
}