Boot up the machine, and after the BIOS screen, hold down the left Shift key. You will then be prompted by a menu.
Hit the down arrow until you the one with the recovery mode in the description and then hit Enter.
Now you will see recovery menu:
Select root in the bottom of recovery menu
Type command:
passwd root
Enter new UNIX password:xxxxxx
Retype new UNIX password:xxxxxx
passwd: password updated successfully