You must first download the pack list by shipment, taking care to select Reset the Box Content Source and Download. That will reset the Box Content to NONE in Seller Central.

Download a Pack List for a Shipment

Upload to Seller Central by navigating to "Manage FBA Shipments" in your Seller Central account and select the shipment. (Menu > Inventory > Manage FBA Shipments)

Identify the shipment for which you want to upload a packlist and select "Work on Shipment."

A new tab will open -- Scroll down to section 3, "Shipment Packing"