You should have a Julia menu entry in the menu bar:
which has a
Settings... entry at the bottom.If you don’t see that press
Ctrl-Shift-P (or Cmd-Shift-P on macs) to bring up the command panel and type in Julia Client: Settings.
Either way you’ll get to the Julia specific settings Juno has – scroll down until you get to the Console Options headline and you’ll find the Fallback Renderer option:
