Your store's default currency can be found in the Shopify admin when you head to Settings > General. A "store currency" section can be found at the bottom of this page. Please note that when you change currencies, your prices do not convert in accordance with any exchange rate—this means a $10 NZD product will become a $10 USD product if you make this change, so you'll need to manually amend your product prices once the currency has been switched.
Please also note that if you have already accepted orders on your store, the option to change your currency will be locked. You will need to contact our support team so that we can authenticate you and allow you to change the currency on the store from our end.
Victor | Shopify Social Care