In your Edison, from the root folder you should go to
root@edison:/# cd etc/wpa_supplicant/
There you'll find the network configuration files. Once there type this:
In that file you should see the networks available for your Edison (including the one you want to forget). Now using a file editor like vi you should be able to remove manually that specific network.
The vi file editor can be tricky at first (in case you haven't use if before), so I suggest to take a look at this website where you'll find a list of useful commands, vi Editor Commands.
vi and I aren't exactly friends, so I used nano.
Thanks so much!