Assuming you mean favicon.ico, you can do what NoahY suggested and put the icon at /favicon.ico.
You should also add a meta tag pointing to the icon:
<link rel="shortcut icon" href="/favicon.ico">
You can add this code in a custom theme. If you don't already have a theme, copy the "Default" folder in qa-theme then create a file qa-theme.php with this content:
<?php
class qa_html_theme extends qa_html_theme_base
{
function head_custom()
{
parent::head_custom();
$this->output('<link rel="shortcut icon" href="/favicon.ico">');
}
}