Every day the locations are revealed at 2pm for the following day. you can only get the free latte between 1-2. Just turn up and ask for it. It doesn't state, but guessing it may include extras free as well like syrups, whipped cream, coconut milks, drizzles etc, as the pictures of all different drinks hint at this. If you're in London you can have a coffee crawl (and hopefully other cities too). Of course, you could give the drink to a homeless person if you don't like coffee/don't want it at the time/irrationally hate Starbucks for being excellent at running a business which changed British high streets for the better.