Increase quality level of radio on the Muso?

The stream quality itself is in the hands of the stream providers.  If you want them to improve . . . maybe write to their customer service reps (if they have any?).

Or do you mean increase the quality of sound reproduction by the Mu-so itself?

Ah, ok thanks. I thought there may have been a quality selector, similar to Spotify, to choose your desired stream rate.

As Bart says, the stream is in whatever form the station chooses to send out, and that's it. There are, though, some stations that put out more than one stream, for example, if you look in 'Naim's Choice' you'll see three different Radio Paradise streams, 128, 192 and 320k. 

Also, keep your firmware up to date, and you'll soon be able to get the new BBC HLS streams in a choice of bit rates which sound great (assuming you're in the UK.)