Manage Channels are found under Brand Settings, this menu will only become available when you're inside a brand.
The brand can be changed from the top menu, now labeled Brand X as an example, so you can switch brands from this menu and access channel options of specific brands' channels.
Facebook Channel Settings
Whenever the password for a Facebook page changes, or even a new admin is designated, a re-authorization of the channel will be needed. Clicking on Reauthorize ("Brand X") will reconnect the channel without deleting any data.
Instagram Channel Settings
On Instagram's channel settings you can set the manual publisher, the person who will be notified when it's time to post. Also, just as in the Facebook settings, from this page, you can remove the channel.
Creative Assets and Twitter settings
These two channels are very simple and work just as Facebook and Instagram's. For Creative Assets, you can modify the channel's name and icon. On Twitter, you can disconnect the channel, as there aren't many actions to perform for Twitter channels.