As designed, you wouldn't as I understand it. That's one of the limitations to this starter solution. I believe you'd need to open up a products based layout where you have access to the inventory level fields and update the amounts there--not how I'd devise an inventory management system.
For an alternative approach that you can consider, (It allows you to not only see what your current inventory levels are, but allows you to see how levels change over time), do a search of this forum for "inventory ledger".
Terrific, I will do that right now.