You can Add or Update special price of the product by the SpecialPrice Interface. Catalog module contains the Special Price interface to update the special price of the product.
Base defination of the update special price of the product,
public function update(array $prices);
New for me using SpecialPriceInterfaceFactory for updating special prices. I have developed the extension for updating any prices in Magento 2 – https://www.magepsycho.com/extensions/magento-2/magento2-mass-regular-special-tier-group-price-importer.html