The "Buy It Now" is considered a dynamic checkout button, and can be replaced by payment method checkout buttons. Unfortunately, Shopify does not allow theme or translation apps to modify the "Buy It Now" button text directly in the theme editor or by translating. For details, you can check the reply of Shopify staff on this matter here.
However, there are ways to implement it under the hood. We provide a small customization for merchants who purchased our theme to change this button text. You can contact us at our Support Portal for more details.