Can external apps interact with Shopify admin API to adjust product quantity?

[ANDev]

From a desktop app on windows, I want to send a message to my shopify admin page to decrease the quantity available for a product.

Is it possible to do so?

I looked into Shopify admin API, but it seemed this api was meant to be used inside the Shopify store's code. I need to access my shopify admin from an external app.

Thanks for any help!