How to change a field value in a product variation when someone edited the order manually?
Is there any event when order items is edited? 


You could use hook_ENTITY_TYPE_update

/*
 * Implements hook_ENTITY_TYPE_update()
 */
function mymodule_order_update(\Drupal\commerce_order\Entity\OrderInterface $order) {
  // your code goes here
}




If you don't want to use a hook, then consider using the Hook Event Dispatcher module.


  This module dispatches events for several Drupal core hooks. This
  allows you to use the D8 Event Subscriber system, instead of the
  outdated hook system, to react on certain events. The module includes
  events for the most common hooks.