Change “Select Options” label in button to “Buy Now!” in store page

    add_filter( 'woocommerce_product_single_add_to_cart_text', 'wpse_woo_custom_cart_button_text' );
add_filter( 'woocommerce_product_add_to_cart_text', 'wpse_woo_custom_cart_button_text' );
function wpse_woo_custom_cart_button_text() {

  global $product;

  $product_type = $product->get_type();

  // To modify on a single product page
  if( $product_type == 'variable-subscription' || $product_type == 'variable' ){
    if( is_single() )
        return __( 'Buy Now', 'my-textdomain' );
    else
        return __( 'Choose options', 'my-textdomain' );
  }
  // To modify on the shop page
  else{
    switch ( $product_type ) {
      case 'external':
        return __( 'Buy Now', 'my-textdomain' );
      break;
      case 'grouped':
        return __( 'View products', 'my-textdomain' );
      break;
      case 'simple':
        return __( 'Buy Now', 'my-textdomain' );
      break;
  // here's your use case
      case 'variable':
        return __( 'Select Value', 'my-textdomain' );
       break;
  // Not a default product type
      case 'booking':
        return __( 'Book Now', 'my-textdomain' );
      break;
      default:
        return __( 'Learn more', 'my-textdomain' );
    }
  }
}

you can put this into your functions.php file