Reset Debian Password

Solution

In the boot menu, edit the default kernel scripts by pressing "e" (select the kernel file you need to edit). Local the line with the keyword starting with "kernel" or "linux", at the end of the line, insert:

init=/bin/bash

Then save and reboot (usually Ctrl-X or F10). You can then mount the hard-drive in the booted kernel, with:

mount -o remount /

And then perform password change with:

passwd

Reference

https://www.itzgeek.com/how-tos/linux/debian/how-to-reset-root-password-on-debian-9-stretch.html