To change the text on the "Add to Cart" button to read "Buy Now" or any other phrase of your choice you can modify your theme's language settings.
First, open the language editor by following these steps.
In the "Filter translations" field, type "Add to Cart" (or, if you've already changed this to say something else or are using a language other than English, type whatever text is currently appearing on your "add to cart" buttons):
Next, locate the "Add to cart" field under the "Products / Product" header:
Change the text currently in that field (by default it will be "Add to Cart") to whatever you'd like.
It's important to note that changing this value will change it on all product pages and quick view windows. If you only want to change the wording of the "add to cart" button on some products, this requires custom development with a Shopify developer.