Solution:
You can use the function wc_update_order() to fetch an existing order object.
$order_id = $_GET[ 'order_id' ]
$order = wc_update_order( array ( 'order_id' => $order_id ) );
// now you got the woocommerce order object and you can execute many functions
//
$address = array(
'first_name' => 'Fresher',
'last_name' => 'StAcK OvErFloW',
'company' => 'stackoverflow',
'email' => 'test@test.com',
'phone' => '777-777-777-777',
'address_1' => '31 Main Street',
'address_2' => '',
'city' => 'Chennai',
'state' => 'TN',
'postcode' => '12345',
'country' => 'IN'
);
$order->add_product( get_product( '12' ), 1 ); //(get_product with id and next is for quantity)
$order->set_address( $address, 'billing' );
$order->set_address( $address, 'shipping' );
$order->calculate_totals();
For a complete list of all the functions you can call on the order object, execute the following code after you fetch your order
var_dump( get_class_methods( $order ) );
This will list all the functions that are available to the Woocommerce Order object.
Hope this answers your question