If you haven't already done so, create an includes/languages/YOUR_LANGUAGE/YOUR_TEMPLATE_NAME folder and copy your includes/languages/YOUR_LANGUAGE/header.php file to it.
Open the file in your favorite editor and find the following line
define('HEADER_TITLE_LOGIN', 'Log In');
Edit 'Log In' to read whatever you wish, taking care to preserve the single quotes before and after it.