06 how to reset root password on Ubuntu 14.04
By default the first user’s account is an administrative account, so if the UI is prompting you for a password it’s probably that person’s user password. If the user doesn’t remember their password you need to reset it. To do this you need to boot into recovery mode.
Boot up the machine, and after the BIOS screen, hold down the left Shift key. You will then be prompted by a menu