Product stock is managed in two different places.
Total stock quantities are managed in the form editor itself, under the question configuration of the product question.
The amount of stock that has been sold can be changed or reset under the form's Results page, which is accessible via the Dashboard.
The "Remaining Stock" shown on this page represents the number of items still available for purchase. This is determined by subtracting the Allocated Stock from the Total Available Stock. When the Remaining Stock reaches 0, the product will be listed as sold out.
The Products screen also gives an overview of how much of product has been allocated, even in a product is no longer on the form.