If you've just sold out of a product and you want to re-list it, you can find it under the "Sold out" filter on your Products page.
Once on the "Sold out" filter, locate your product and click the title or select "Edit" from the Actions dropdown.
You'll be taken to the "Edit" view, where you can scroll down to the "Quantity" section and update the number available.
Once done, scroll to the bottom of the page and click the green "Publish" button.
You'll now see the product on the "Active" filter of your Products page.