It’s this folder:
yoursite.com/public_html/wp-content/plugins/wptouch/themes/default/comments.php
So if you’re using some sort of file or ftp client software, like FileZilla for example. You log in and then click on your public_html folder, then that opens and you click on your wp-content folder. When that opens you click on the plugins folder, then when that opens find the wptouch folder, then themes, then default. The comments.php file should be in there.
Download that file, make the changes and re-save.