Magento 2 :: root script

If document root pub:

ini_set('display_errors',1);
ini_set('memory_limit','160000M');
ini_set('max_execution_time', 0);

use Magento\Framework\App\Bootstrap;
use Magento\Sales\Model\Order;
require __DIR__ . '/../app/bootstrap.php';
$params = $_SERVER;
$bootstrap = Bootstrap::create(BP, $params);
$obj = $bootstrap->getObjectManager();
$state = $obj->get('Magento\Framework\App\State');
$state->setAreaCode('frontend');
$objectManager = \Magento\Framework\App\ObjectManager::getInstance();

// state you want
$orderState = Order::STATE_CANCELED;
// status you want
$orderStatus = Order::STATE_CANCELED;
// order id you want
$orderId = 72395;


$objectManager = \Magento\Framework\App\ObjectManager::getInstance();
$order = $objectManager->create('\Magento\Sales\Model\Order')->load($orderId);
$order->setState($orderState)->setStatus($orderState);
$history = $order->addStatusHistoryComment('Order Was Set To '.ucwords($orderStatus).' Programmatically by Developer .', $order->getStatus());
$history->setIsCustomerNotified(false);
$order->save();
echo "Order Increment Id = ".$order->getIncrementId().'<br>';
echo "Order ID ".$orderId.' Status Changes Successfully';

if document root not pub:

ini_set('display_errors',1);

use Magento\Framework\App\Bootstrap;
require __DIR__ . '/app/bootstrap.php';
$params = $_SERVER;
$bootstrap = Bootstrap::create(BP, $params);
$obj = $bootstrap->getObjectManager();
$state = $obj->get('Magento\Framework\App\State');
$state->setAreaCode('frontend');
$objectManager = \Magento\Framework\App\ObjectManager::getInstance();
$registry = $objectManager->get('Magento\Framework\Registry');

$categoryId = 3;
$category = $objectManager->create('Magento\Catalog\Model\Category')->load($categoryId);
echo $category->getName();