how to modify the confirmation email in Ubercart?
You'll want to override the file 'uc_order-customer.tpl.php' found in the ubercart module. The way you do this is:

You need to copy two files from sites/all/modules/ubercart/uc_order/templates
The files to copy are uc_order.tpl.php and uc_order-customer.tpl.php

Then just edit uc_order-customer.tpl.php to your liking.