If they are currently connected to a WiFi network, you can edit the file over SSH.
Running sudo nano /boot/octopi-wpa-supplicant.txt will allow you to edit it. You can duplicate the network blocks to add additional networks, so when you move it the Pi will choose the first available.
I haven't used SSH before, would you mind breaking down the steps? So far it looks like you use something like PuTTY, connect to the IP address... and then send that command in the portal. Am I on the right track?
I have jobs running at the moment, otherwise I'd have jumped in already and tried it.
That's one way - check this guide out to see another
and since you never used ssh a little hint in advance:
when your'e done with editing the file press CTRL + x to close the texteditor.
I will ask you then if you want to save the file which you can approve with y followed by enter.