Add Heap Analytics code to HTML
All of the HTML that Craft uses to render your site lives in your craft/templates folder.

In one of the files in that folder, you're presumably defining your &lt;head&gt;&lt;/head&gt; tags.

You'd want to find where that is and insert your JavaScript analytics tracking code before the closing head as per the Head Analytics installation docs.

Probably worth a read through the Craft does to familiarize yourself with Craft concepts as well.