@Neon - The "Buy Now" button is actually an "Accelerated Checkout" button. Check out this documentation that walks through what is shown (ie. Buy Now, G Pay, Apple Pay, etc...) and under what circumstances: https://help.shopify.com/en/manual/payments/accelerated-checkouts. Long story short, there is no way to permanently change the button text to "Buy Now" for all use cases nor would we recommend it :)
Hope this helps!