For users in the UK/EU
Currently cardholders in the UK/EU can change their PIN code from any Visa ATM. Those same cardholders can also view their existing PIN after they go through verification in the Card setting screen under the "View PIN" menu item.
For users in the US
Currently cardholders in the US can set their PIN code when they activate their card and thereafter change their PIN via the "Change PIN" feature in the Card setting screen. You are unable to view your PIN so please try to memorize it or write it down in a safe area.