Yes, it is. Assuming your theme’s files have the correct permissions you can edit the header.php file (most likely containing your header section) directly from the administration. Just go to Design > Theme Editor. This will pull up the theme you are currently running in your blog. Click “Header (header.php)” to edit the file. If you can edit you’ll see an “Update File” button below the text area. If you see a message (“You need to make this file writable before you can save your changes. See the Codex for more information.”) then the file does not have the permissions to edit through the admin and you’ll have to edit the file directly. If you see the message go ahead and click the Codex link and read up on it.