Thank you for reaching out with your question. To achieve the outcome you're looking for here, you'll want to consider some of the many currency apps available on the Shopify App Store. There are plenty to choose from, although I do know the Bold Multi-Currency app will allow you to set your own manual currency conversions, as opposed to real-time exchanges rates being pulled. The app should also allow you to determine which currency will show depending on the customer's location.
I'd suggest taking a look around at the different options available to you but this is a good place to start. Please let me know if you have any further questions.
Victor | Shopify Social Care