The main demo site uses the Currency Switcher for WooCommerce plugin from Aelia within the top bar. The plugin produces a shortcode which you add to a widget. The main demo includes it within this widget area:
Appearance > Widgets > Top Bar Right
It’s the only premium plugin used on the demo site as it’s very well coded and the support from the developer is excellent.
If budget is an issue and you want a free plugin there are a number of options. The WordPress plugin directory includes these: