Navigate to the "Settings / Language Switcher" section of your langify backend:
Select the theme you want to edit the language switcher for.
If you scroll down a bit, you will see the "Select a language switcher" section.
Simply click "Select" next to the switcher you wish to use.
Note that you can change the width, position, etc. via CSS as explained here: http://support.langify-app.com/support/solutions/articles/1000186503.