Use LiveCD and chroot to linux system ...
now use passwd for change password .
Either you can boot with single user mode then change root password
or, boot system by Live CD and change passwd
sudo passwd root changes your root password !
You must write the above command when you access to the terminal when you reboot by ubuntu or
single entering in grub.
You can boot through the Linux and change password in its terminal.
You can when system boot edit grub
and edit line then kernel fisrt writen to line
edit this line and end of line write single
and click "b" to boot system in runlevel 1
your system boot in single user and can change password without pass password