You can access the settings for each channel by going to Channels, tapping on the one you are interested in, then tapping Settings (note that not all channels have settings).
You can also access each channel’s settings right before publishing a product. After filling out the product card, tap “Next” and you’ll be brought to the Select Channels page. When you tap on the name of a channel you’ve already logged into, you’ll be brought to the channel settings page. Once you’ve finished editing the settings, tap Save and you can pick up right where you left off on the Select Channels page.