Go to the dashboard.
Originally Posted by jmissoula
Expand the "Appearance" tab on the left
Click on "Editor"
you should see (depending on theme)
Select that file
You should then be editing the header.php file, Look for a line like:
Insert your custom header data directly below that line.
<meta http-equiv="Content-Type" content="<?php bloginfo('html_type'); ?>; charset=<?php bloginfo('charset'); ?>" />
If you change themes you have to repeat the above.
In the alternative you can use a plugin like All in One SEO and it has a place in the plugin options settings to insert custom headers.