Change the product status after checkout_complete I have one 'select' field in product page it has value 'Open', 'Closed'.I want to changed to 'Closed' after completing the first order of this product. 

There is one issue which someone answered for Drupal 7 and it is done by using 'Rules' module.
How I can change the value of a commerce product field after Completion process?

How I can do that using Hooks and without Rules module?

EDIT:
I feel we can use below hooks. But how we can find the product and change the custom field value of the product? 

function hook_commerce_checkout_complete($order) {
  if($order-&gt;status == 'completed') { 
  }
} 


hook_commerce_checkout_complete

EDIT 2:

Got the solution for Order Complete event - https://medium.com/@sumanthkumarc/writing-event-subscriber-for-order-complete-event-in-drupal-8-commerce-2-x-196292330b1c. Now question is How I can modify the product custom field value?

public function orderCompleteHandler(WorkflowTransitionEvent $event) {
  /** @var \Drupal\commerce_order\Entity\OrderInterface $order */
  $order = $event-&gt;getEntity();
  $items = $order-&gt;getItems();
}


How can i get the product id from the order items and change the custom field(field_prodstat) value
You'll want to use the commerce hook_commerce_checkout_complete which runs after a order is complete (AKA paid for)

http://www.drupalcontrib.org/api/drupal/contributions%21commerce%21modules%21checkout%21commerce_checkout.api.php/function/hook_commerce_checkout_complete/7

That provides the order and you'll just need to get the product ID from that $order variable, load it with product_load(), change the value you want and then save