How do I programmatically update the order status? After Payment is accepted, I need to update the order status to "Completed" from the page controller. 

How do I do this, on Drupal 8?

I find no documentation about this; there is an example for Drupal 7.

// Updating Order status.
$orderObj = commerce_order_load($orderID);
commerce_order_status_update($orderObj, 'canceled', TRUE, 'Order Voided');
$orderObj-&gt;log = 'Order cancelled programatically';
commerce_order_save($orderObj);
You want to use the State Machine methods of the Commerce Order class. 

It would be something like this, but it will depend on what you called your order states.

$order_state = $orderObj-&gt;getState();
$order_state_transitions = $order_state-&gt;getTransitions();
$order_state-&gt;applyTransition($order_state_transitions['complete']);

$orderObj-&gt;save();


If you aren't sure, you can always check the $order_state_transitions array to see what options you have.