How to add Favicon

either just add favicon.ico to your root folder
or
inside your header.php add (assuming your favicon is in your theme folder):

<link rel="icon" href="<?php echo get_stylesheet_directory_uri() ?>/favicon.ico">

or
inside your functions.php (assuming your favicon is in your theme folder):

add_action('wp_head', 'my_head');
function my_head()
{
echo '<link rel="icon" href="'.get_stylesheet_directory_uri().'/favicon.ico">';
}