Transmission-daemon keeps resetting

Transmission daemon re-writes the configuration file with the old options when it exits. To make your changes persist, you need to edit the configuration file without the deamon running. So first:

sudo stop transmission-daemon

Or if you did not install it via Software Center / Official repositories:

sudo killall -HUP transmission-da

Afterwards you can edit settings.json as you would expect and then restart transmission:

sudo start transmission-daemon