Live channels while not installed by default can be installed via the play store,
There are 2 options to use it as of right now from what I can find
-You can use button mapper to open the app from a unused button such as netflix
Or
-use a third party launcher
You can do the same for the play store and it will allow browsing just as on older versions of Android tv such as seeing apps you own, seeing a full screen wall of apps to download (instead of the single row shown in the new launcher), and better searching capability
Other 3 ways I've found:
Click on voice search button and say "Live Channels".
Go to system settings, then apps and open from there.
Install "PVR Live" from the Google Play Store and it will create an icon that you can click and launch the Live channels app. You might get a message telling you to use the "Live Channels app" but ignore it. You pretty must want it as a shortcut to launch the Live Channels app.