Shift while booting (Esc for v9 earlier version);
Highlight the normal booting item (which is highlighted by default);
’e’ to edit;
Find the line starts with
quiet splash $vt_handoff …,
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.
WARN tip: Desktop version may prevent root to login even if the password is correct, so a sudo-abled non-root user is a good choice to login.
https://help.ubuntu.com/community/LostPassword The Other Way