How can I update Inventory record in Zoho Creator Subform

How can I update Inventory record in Zoho Creator Subform

Hey there,
I am trying to build the app for Inventory Management in a subform. I want to update stock selected in a subform to be subtracted from my master inventory in terms of quantity. How can I achieve this in a subform using Workflow. 

Any help would be appreciated!

Thanks in advance!