In order to reset the password, you need to boot your debian installation into single user mode. You can do this by following the steps below;
1. Once grub menu comes up, select the single user entry and press ‘e’
2. Select the kernel … line and press ‘e’ once again
3. Append ‘init=/bin/bash’ at the end of line and press b to boot
You will be dropped into a shell, next you need to mount the filesystem in read/write mode in order to change the password.
5. mount -n -o remount,rw /
6. passwd
7. reboot
That’s all there is to it!