The woocs_update_rates_wpcron action is the scheduled event FOX registers for automatic exchange rate updates. The plugin attaches its own rate refresh to it, so firing the action runs a full update immediately, exactly as the scheduler would.
That makes it the right way to trigger a refresh from a real server cron job. WordPress cron only runs when somebody visits the site, so on a low-traffic shop the scheduled update can drift for hours; a system cron calling WP-CLI removes that dependency.
do_action('woocs_update_rates_wpcron');
From the command line, which is the usual way to wire it to a server cron:
wp eval "do_action('woocs_update_rates_wpcron');" --path=/var/www/example.com
Parameters: none.
Note: the update contacts the configured aggregator, so it takes as long as that request takes. Do not attach it to a hook that runs on every page load.