The Header/Admin password is stored in the header.inc.php file. In order to reset it, you should open the header.inc.php file, located at your eGroupWare root directory, find the following line:

$GLOBALS['egw_info']['server']['header_admin_password'] = 'MD5 Encoded
Password';

and replace it with

$GLOBALS['egw_info']['server']['header_admin_password'] = '';

This way you will be able to access the Header/Admin part of your eGroupWare with a blank password.