Navigate to the live event you want to make free under Content > Live Events. Under Pricing select Free for Everyone.
There is one caveat to setting a Live Event as free, it still requires the user to checkout. There is a small workaround for this, set the Live Event as "Require Access" and add the Live Event to a Freebie Bundle, that way when a user clicks on the Live Event to get Access they will instead be able to claim the freebie bundle without going through checkout.
It's also important that users are signed in when viewing a live event even if it's free so that they can participate in the live chat.