1、代码

public function __construct(
    \Magento\CatalogInventory\Api\StockRegistryInterface $stockRegistry
)
{
    $this->stockRegistry = $stockRegistry;
    parent::__construct();
}
public function yourMethod() {
    $sku = 'ABC123';
    $qty = 10;
    $stockItem = $this->stockRegistry->getStockItemBySku($sku);
    $stockItem->setQty($qty);// 设置库存数量
    $stockItem->setIsInStock((bool)$qty); // 启用库存
    $this->stockRegistry->updateStockItemBySku($sku, $stockItem);
}