/file/print does show the .npk file on disk1, but I don’t know how to tell the system to use this file instead of trying to download the file from the internet.
This system has no connection to the internet, so the normal upgrade process fails.
Usual mode of manual upgrading ROS is to copy npk file to root of device’s storage. After that reboot device and it should pick the file.
The trick in your case is how to move/copy file from flash drive to device’s storage. I don’t think there’s command to actually copy file from one directory (or mounted filesystem) to another inside ROS though. You might get some inspiration for a workaround from this old thread.