It is not uncommon for customers to make a mistake while generating an order on Shopify, and then contact you to correct that. Unfortunately, due to Shopify rules, we cannot write back altered data to an existing Shopify order. The only thing that is changeable on such an order are comments. The proper course of action for a scenario where the customer needs to reduce their ordered quantities is to delete that order on Shopify and generate a new one entirely. Similarly, if a customer needs to increase quantities or add new items, that will need to be on a completely new order due to these Shopify restrictions.