Observer

In order to redirect a customer from observer you need to use another way than the Controller 

protected $_response;

protected $_objectManager;

public function __construct(
    \Magento\Framework\App\ResponseInterface $response,
    \Magento\Framework\ObjectManagerInterface $objectManager,

    ) {
        $this->_response = $response;
       $this->_objectManager = $objectManager;

   }

then in your function you can use this 

Step 1: Create new column and set value for the existing order

Firstly, you need to create a new column in table sales_order in your installer file 

 $setup->getConnection()->addColumn($setup->getTable('sales_order'), 'order_attribute', 'varchar(1)');

In this post, we named it “order_attribute”.

Step 2: Create extension_attributes.xml file