To change the level of the actual recording, one would have to adjust the gain level on their audio interface/USB microphone/from their computer's system preferences.
(On Mac: Open System Preferences > Sound > *click 'Input' at the top of the window* > *adjust the slider next to 'input volume' accordingly*)
(on Windows: Select Start , then select Settings > System > Sound . In Input, ensure your microphone is selected under Choose your input device, then select Device Properties. On the Levels tab of the Microphone Properties window, adjust the Microphone and Microphone Boost sliders as needed, then select OK.)
To adjust the levels of a participant's volume mid-meeting (not what's on the recording) the host/producer(s) can drag the fader under any participant's name in the 'Participants' window: