You can alter or add the Expiration date on the Box Contents page.
Click directly on the expiration date (or N/A) and it will turn into an editable field to update
Your expiration dates will be correct when you transmit boxes.
Batch already closed? No problem!
If your shipment is still in Working status, you can get back to your Box Content Information page by clicking the 3 horizontal lines to the right of the shipment under Working Shipments and selecting Box Content Information
Once all of the needed changes have been made, click the Transmit Updated Boxes button in the lower right hand corner to resend your Box Content Data to Amazon.