Not like other Linux systems, the "single user mode" does not help reset the root password because Debian asks for root password even in the single user mode.
I found this post which gives me some hint. Here is what worked for me:
On gurb menu, press "e" to enter edit mode
Go to the line starting with "linux", and add " init=/bin/bash" to the end of the line.
Ctrl-x to boot, and you will see the # prompt
Type "mount -o remount rw /" to remount the root file system as writable.
Use command "passwd" to change the root password or any user's password
Reboot by "reboot" command