Cal here from Shopify, thanks for reaching out to us. Have you made your first sale? If so, you will need our assistance in changing your currency, and you won't be able to do it manually. Are you looking to switch your store back to USD?
This feature is locked in the admin if you have already made sales, even a test order. The reason for this is so you can reach out to our support team and chat with us about the effects this will have on your business if you were to change currencies after sales have been made. It can have an impact on things like reports and apps, so we always like to discuss the impact prior to the merchant switching. Are there any apps on the store that the currency switch might impact? You will need to manually adjust all the prices of your products on the store again. Are you ok with this?
If so, please share your MyShopify URL with me here, and I will email the account owner on file with further instructions on how they can adjust the store currency.