If you want to set price update while changing the quantity in product detail page then paste below script

<script> 
require(
[
 'jquery', 'Magento_Catalog/js/price-utils'
],
function($, priceUtils) {

 $(document).ready(function(){ 
 
 $('#qty').on('input propertychange paste', function() {
 changeCuPrice();
 });
 function changeCuPrice(){ 
 var totalQty = $('#qty').val();
 if(totalQty == 'undefined' || totalQty == ''){
 totalQty = 1;
 } 
 
 var price = $('.price-final_price .price-container .price-including-tax').data("price-amount");
 totalQty = parseFloat(totalQty); 
 
 var flprice = parseFloat(price) * parseFloat(totalQty);
 var inPrice = priceUtils.formatPrice(flprice);
 $('.box-tocart .price-final_price .price-container .price-excluding-tax .price').text(inPrice); 
 }
 
 setTimeout(function(){ changeCuPrice(); }, 500);
 
 });

});
</script>

at this path

app/design/frontend/Vendername/Themenamge/Magento_Catalog/templates/product/view/addtocart.phtml