I assume you are talking about the stand alone version of Kontakt, since as a plugin it uses your DAW for output channels.
In the stand alone version if you open the Kontakt output panel and click on the output fader, the dialog for routing output should come up. Selecting the output should show all available output channels. If it doesn't show all channels you'd need to adjust something with your audio driver settings. Here's what mine looks like with my Focusrite Saffire which has 10 output channels.
As for switching back and forth, for each instrument loaded into Kontakt you can set what MIDI channel it receives and where it's output is sent to (Kontakt has it's own mixer) The way you "switch" between instruments is by using different MIDI channels to direct the MIDI notes to the correct instrument. Might want to check NI and YouTube for videos on how this works in Kontakt, once you see how it works you shouldn't have much trouble.