Products & Listings > Product Sync > Link Quantities
Navigate to the 'Product Sync' option on the side menu, under Products & Listings and then select Link Quantities.
Then, on this page, click the link icon next to the product you wish to update or un-link:
A pop-up will then appear offering you two options:
To update, enter the quantity in the box in the bottom-right and click Update.
To un-link any, or all, products, tick the boxes next to the products you with to un-link and click Unlink.