You need to contact Shopify Support for this to happen since you made your first sale. You can access the Live Chat here: https://help.shopify.com/en/questions#/contact
Cal here from Shopify, thanks for reaching out to us. How are you doing today?
If you're looking to change your store currency, and you have not yet made a sale, you should be able to go to Settings > General > Store Currency, and make the switch.
Let me know if you have any issues finding this. If you are still struggling to alter your currency I'd be happy to assist further, what is the MyShopify URL for the store?