With the store website loaded, navigate to All Products, and use the search or the scrolling list to find the product whose variants you need to edit.
Select the variants you wish to edit:
The variant should say N/A for its quantity:
Scroll right on the scrollbar at the bottom of the variants list, scroll back up, and click “edit” next to the selected variant.
Uncheck “track quantity”:
Scroll down and click save.