Before starting a recording, click Audacity > Preferences. Under the Devices tab will have the option to switch the audio channels between Mono and Stereo.

If you are already working with an audio file, use the dropdown menu in the Track Control Panel (located at the left side of your track) to change the name or sample rate, determine how the track is displayed, split a stereo track to single left, right or mono tracks or swap stereo channels.