How to Reset Root Password On Ubuntu 18.04 / 16.04 / 17.10 LTS
1 Reboot your computer.
2 Hold Shift during boot to start GRUB menu.
3 Select advanced options for ubuntu
4 Select the option with recovery mode
5 Select root and press enter
6 Give command - mount -n -o remount,rw /
7 Press Enter
8 Give command - passwd your_username
9 Give new password
10 reboot
همه توضیحات ...