To change the store currency of your Shopify store, you have to navigate to the Store currency section of the Store details settings page.
If you have already made sales through your store, then you need to contact Shopify Support for them to do this on your behalf. You can read more on how to do so in Shopify's Help Center here.
You can change the billing currency of your Gelato account as well. Read more on how to do so in our article How can I change the currency in my Gelato account?.