woocommerce/templates/single-product/add-to-cart/simple.php

<?php
/**
 * Simple Product Add to Cart
 */
 
global $woocommerce, $product;

if( $product->get_price() === '') return;
?>

<?php 
	// Availability
	$availability = $product->get_availability();
	
	if ($availability['availability']) :
		echo apply_filters( 'woocommerce_stock_html', '<p class="stock '.$availability['class'].'">'.$availability['availability'].'</p>', $availability['availability'] );
    endif;
?>

<?php if (!$product->is_in_stock()) : ?>
	<link itemprop="availability" href="http://schema.org/OutOfStock">
<?php else : ?>

	<link itemprop="availability" href="http://schema.org/InStock">

	<?php do_action('woocommerce_before_add_to_cart_form'); ?>
	
	<form action="<?php echo esc_url( $product->add_to_cart_url() ); ?>" class="cart" method="post" enctype='multipart/form-data'>

	 	<?php do_action('woocommerce_before_add_to_cart_button'); ?>

	 	<?php 
	 		if (!$product->is_downloadable()) 
	 			woocommerce_quantity_input( array( 'max_value' => ($product->backorders_allowed()) ? '' : $product->get_stock_quantity() ) ); 
	 	?>

	 	<button type="submit" class="button alt"><?php _e('Add to cart', 'woocommerce'); ?></button>

	 	<?php do_action('woocommerce_after_add_to_cart_button'); ?>

	</form>
	
	<?php do_action('woocommerce_after_add_to_cart_form'); ?>
	
<?php endif; ?>