Delete a root account a ext3 filesystem

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http://superuser.com – I have downloaded a file from this webpage: http://sourceforge.net/projects/mln/files/mln-templates/Debian-4.0/ This file is a template of a Debian 4.0 operative system, also the type of this file is ext3. The problem is that the username provided in the application official website is wrong, and therefore i am unable use this template. My question is the following, how do i delete the current username and password and replace it by one of my choosing? Do i have to edit the ext3 file to do this? Thank you for your time. (HowTos)