From this "how to reset dnn user password" article we mentioned how to reset user password from a SQL store procedure. Here’s another method we’ll share to people who have trouble with password management.
Download this file
unzip it and upload to your dnn website root, then browse to it, you will have the option to create another super user, once created you just login with this new super user and change the default user password. Done
Thanks to the author of this website: http://www.evotiva.com/Modules/LetMeIn.aspx