At first grub boot prompt, you have 4sec to pres: [e]

grub-menu.JPG

Does not matter which grub kernel you choose as we are not really modifying the kernel.

With your [e] key just choose to edit the first one

add at the end the word "single", like so:

<YTABLE=us crashkernel=auto rhgb quiet single

centos-kernel-edit-to-single-user.JPG

and press [enter]

Back at the Grub menu press [b] to boot.
When the prompt comes up we are in single user mode

centos-single-user-mode.JPG

In this mode we are not required to authenticate and we can go ahead and reset the password by typing at the prompt:

[[email protected] /]# passwd

setting-root-passwrod-centos.JPG

retype the password to confirm and the root password has been changed,

do a shutdown by typing:

shutdown -r now