I'm new to Ubuntu.
How do I get to
/etc/NetworkManager/system-connections/CONNECTION_NAME and edit it? I cant find it.
Open a Terminal with Ctrl+Alt+T and type:
gksudo thunar /etc/NetworkManager/system-connections/
Type your password (you won't get any visual feedback) and again Enter.
This will open a file manager window (Thunar in the case of Xubuntu) with root permissions, so be cautious, which contains the configuration for every network connection that this system has ever connected to. Double click the file you need, to edit.
The reason I used
gksu is because only root has edit permissions, in case you want to modify or restore connection configuration files.
CONNECTION_NAME is a placeholder and can be the name of the connection that interests you most.