Hey there @bing1106 – ScruffR is right, we absolutely recommend performing device updates for manufacturing over USB, hardware serial, or JTAG over-the-wire, not over Wi-Fi.
You’re right–not all products have exposed USB ports, but all circuit boards (including toasters!) require testing before being mounted into an enclosure or housing. In the event that you are using full Photon PCBs in your product, you should simply update them before they have been soldered to your carrier or host PCB, or after they have been soldered to your host PCB, before that PCB assembly has been integrated into the enclosure.
Whatever PCB you are designing will require basic testing at scales of 1K+ units, and that is the most effective, reliable, and appropriate time to update the system and user firmware on your devices.
We do recommend doing end to end functional testing of your product to ensure that they can be properly configured with Wi-Fi credentials and connected to the Cloud, but that is more appropriate at the final inspection and testing stage since any additional assembly or manufacturing steps could conceivably change the performance of the device.
Hope that helps a little bit!