To manually update MF inventory, click the inventory tab on your manage listing page.
From there you can manually adjust quantity by listing.
Once you've updated the inventory on one item you'll see a large yellow banner, continue to update as many listings as you want. When you're done you will see all edited inventory in a red box:
In the yellow banner, you'll have the option to Revert or Save your changes, along with the option to add a note. This is great to use to remind yourself, or let other users know, why you edited the quantity for this item. For example, "Adding 1 QTY because I found one under a shelf" etc,.
If you have any questions about manually adding MF QTY please reach out to support!