WooCommerce Button Text on Archive page.

add_filter( 'add_to_cart_text', 'freelancedev_custom_product_add_to_cart_text' );            // < 2.1
add_filter( 'woocommerce_product_add_to_cart_text', 'freelancedev_custom_product_add_to_cart_text' );  // 2.1 +
 
function freelancedev_custom_product_add_to_cart_text() {
    return __( 'Buy Now', 'woocommerce' ); 
}