In your theme go to the checkout folder and edit success.phtml
If you do not have a success.phtml in your theme then to overwrite this you can copy the one from the base magneto theme folder which can be found
app/design/frontend/base/default/template/checkout
copy the file to your theme in the same directory
app/design/frontend/your/theme/template/checkout
Do any edits you need to the new success.phtml file placed in your theme and remember to flush the magento cache once you have done this
Your code should look like this
<?php
$orderId = Mage::getSingleton('checkout/session')->getLastOrderId();
$order = Mage::getModel('sales/order')->load($orderId);
?>
<!-- Event snippet for Final Page conversion page -->
<script>
wtag('event', 'conversion', {
'send_to': 'AW-23232323223232/23232323233434343443',
'value': <?php echo $order->getGrandTotal() ?>,
'currency': 'INR',
'transaction_id': '<?php echo $orderId ?>'
});
</script>