Hi, I would like to modify NetworkManager.conf before flashing a device.
Reason being that I would like to have my own default connections, i.e disable the auto-default property.
In other words I want to provision devices with connection profiles for eth0 and so on without network manager creating e.g ‘Wired Connection 1’ and so forth.
How could this be achieved using tools to configure the downloaded image? 
Hi, and welcome to the community.
You can add network configuration to an image before programming by adding to the /mnt/boot/system-connections directory. You can use the balenaCLI tool, for example with the balena os configure /path/to/image --system-connection /path/to/local/system-connections.
You can also use the dbus NetworkManager API from a running container to further configure NetworkManager.
The balenaOS network configuration documentation can be found here.
Let us know if that helps.