Reset root password

1. Reboot to Grub Menu (pressing <shift> key)
2. Edit Grub Menu: Press e to edit
3. Alter boot menu

    1FROM: 
    2
    3linux     /boot/vmlinuz-4-4.0-22-generic root=UUID=43ad24d3-e\
    4c5b-44ee-a099-a88eb9520989 ro  quiet splash $vt_handoff CHANGE TO: linux     /boot/vmlinuz-4-4.0-22-generic root=UUID=43ad24d3-e\
    5c5b-44ee-a099-a88eb9520989 rw init=/bin/bash
    6
    7TO: 
    8
    9linux     /boot/vmlinuz-4-4.0-22-generic root=UUID=43ad24d3-e\
   10c5b-44ee-a099-a88eb9520989 rw init=/bin/bash
   

4. Partition is read-write mounted: mount | grep -w /
5. Change password: passwd
6. Reboot system: exec /sbin/init