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:
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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 |