Programmatically update customer data in Magento 2

Using Object Manager

$storeManager = $objectManager->get('\Magento\Store\Model\StoreManagerInterface');
$websiteId = $storeManager->getStore()->getWebsiteId();
 
$objectManager = \Magento\Framework\App\ObjectManager::getInstance();
$customerRepository = $objectManager->create('Magento\Customer\Api\CustomerRepositoryInterface');
 
// load customer info by email
$customer = $customerRepository->get('[email protected]', $websiteId);
 
// load customer info by id    
$customer = $customerRepository->getById(1);
 
// Update customer Info
$customer->setFirstname('John');
$customer->setLastname('Smith');
$customer->setEmail('[email protected]');
 
$customer->setCustomAttribute('attribute_code', 'attribute value');
 
$customerRepository->save($customer);

To get the attribute value you can use this code

$customer->getCustomAttribute('attribute_code')->getValue()