The bitrate controls are accessible via a URL parameter, along with width/height. Something like https://VDO.Ninja/?view=yyyyy&bitrate=10000 will let the viewer request different bitrates. The value is in kilobits per second and the default bitrate is 2500.
The viewer sets the bitrates generally, although you can set maximum allowed bitrates as the publisher of a stream. See the advanced settings in the wiki for more help here.
Bitrate controls work best in Chromium-based browsers. Firefox and Safari have limited support.
You can improve audio quality in the same way, by increasing the &audiobitrate, but you can get better results by just disabling noise and echo cancellation instead.