https://help.ubuntu.com/community/Grub2

1. Power up your computer.
2. Press ESC at the grub menu.
3. Press e for edit.
4. Highlight the line that begins “Ubuntu, KERNEL ………” and press e.
5. Go to the end of this line, add rw init=/bin/bash or add single and press enter.
6. Press b to boot your system.
7. Your system will boot up to a passwordless root shell.
8. Type in passwd username where username is the username you lost the password for.
9. Set your password.
10. Type in reboot.