Try following way:
<pre>
public function __construct(
    \Magento\Catalog\Api\ProductRepositoryInterface $productRepository,
    \Magento\Catalog\Model\Product $product,
    \Magento\Framework\Registry $registry
) {
    $this->productRepository = $productRepository;
    $this->product = $product;
    $this->registry = $registry;
    
}
</pre>
Now
<pre>
$this->registry->register('isSecureArea', true);
$product = $this->product->load(1);
$existingMediaGalleryEntries = $product->getMediaGalleryEntries();
foreach ($existingMediaGalleryEntries as $key => $entry) {
    $entry->setLabel('BlahBlah');
}

$product->setMediaGalleryEntries($existingMediaGalleryEntries);
$this->productRepository->save($product);
</pre>