Hey Lydia,
You may use the wp_head action to add your favicon link in the head of your website. Here is a example code snippet which you may add to your child theme functions.php:
add_action( 'wp_head', 'example_head_extra' );
function example_head_extra() {
?><link rel="Shortcut Icon" type="image/x-icon" href="http://www.getbeans.io/wp-content/themes/tm-beans/lib/favicon.ico"><?php
}Note that it is extremely rare that you would create files such as header.php to add, overwrite or remove thing. 99% of the time, you will use actions and filters or Beans API functions in your child theme:-)
Have fun,
Many thanks! That was exactly what I needed. 🙂