If you need to reset the root password for Debian you can follow the steps below
Boot the GRUB menu and press "e" and the end of the line of "kernel... " after quite insert the following code
init=/bin/bash
boot the os by press "b"
and the enter the code
mount -n -o remount,rw /
Then you can change the root password by
passwd
Reboot the OS
Cheers