There’s not really a way to bulk update stuff using the Shopify API. They usually work on single items at a time (e.g.: get single product, update single product).
It’s best if you can actually export a CSV of all the products in a Shopify store, then manually change them, and export them back, or use a bulk editor UI, like Matrixify - Matrixify - Bulk Import Export Update Migrate | Shopify App Store.
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