You'll need to boot into single user mode.
Restart the machine, and go into grub boot loader, may need to press escaoe at the prompt to get to the menu.
At the grub menu select the correct linux boot, press 'e' to modify the boot parameters.
Highlight the line item beginning with 'kernel' press 'e' to edit.
Append ' 1' to the end of the line.
Press 'b' to proceed with boot into single user mode.
You can now use 'passwd' to change root password. shutodwn and reboot.