Multi-Currency does not have the ability to convert currencies in Shopify's checkout by itself. Due to security concerns, Shopify doesn't provide access for third-party developers to change elements of their checkout process. You're only able to set one, default currency to process payments with on a Shopify store.
Multi-Currency integrates with Bold Cashier to provide your customers with the flexibility to pay in their desired currency. Cashier uses its own payment gateway providers to accept payments for orders in currencies that are not the store's default.
Visit Multi-Currency & Cashier Integration for more information on how to set up this feature.