I assume the device does have a filesystem on it? Or is the content hard baked into the device’s boot image? If the latter, then you’d have to make a new boot image with the changed file.

assuming there is a filesystem on the device, you’d have two options: either use a serial terminal emulator to connect to the device and use an editor running on the device to edit the file; or use a file transfer program to fetch the file from the serial device onto your big computer, edit it there, and then send it back the same way.

in the old days I would have used kermit for the transfers because it was the very best and supported on many different platforms. But it depends what serial file transfer protocol is supported by your device.

I remember kermit!! Yes, I have to use special tools made for accessing these devices. The do have a file system, but they do not enumerate it.