Click on the Audio Track dropdown menu and select the track you want to go to your destination.
Passthrough is the default option. This will stream out all the channels if your destination accepts multiple audio channels.
Next to each Track is the Packet Identifier. This is used to differentiate audio streams that are being transmitted within the same transport stream.
The track can be switched while the Destination is Active. Go back into the Destination settings, select your new track, and press Save.
It can take 30-40 seconds for the switch to happen.
Click here to learn more multichannel audio for streams.