You can change currency from the Event Overview page for Lightning Events by clicking the Currency Picker and selecting a currency. The page will refresh and display the selected currency for ticket prices and for other prices in Event Registration.
You can also change currency in the same way for Community Events.
If you change your currency while tickets are in your Shopping Cart, you will receive a warning that your cart will be emptied. If you confirm the change, your currency will change and your cart will be emptied. You will have to register for your event again.
You are unable to change the currency from Event Registration. The Currency Picker will not open when clicked.