Shift while booting (Esc for v9 earlier version);
Highlight the normal booting item (actually, it is highlighted by default);
'e' to edit;
Find the line starts with "linux ....";
Edit "ro" (read only) to "rw", and add "init=/bin/bash" to the end of the line;
Ctrl+x or F10 to boot using this modified temporary item;
Root shell will be available, passwd to modify and then reboot.
https://help.ubuntu.com/community/LostPassword > The Other Way