Updating table within the app for customers

The table in my database has columns: _id, picker_id, order_num, item, qty_left, qty_picked
First, I would like to input values for picker_id, order_num, and item. All three of these criteria need to match to return the _id. Then, with the qty_picked input of the user, qty_left should be updated. [qty_left = qty_left - qty_picked]
I am having trouble implementing this and using the updateservice
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