Change the forgotten root password.

Reset forgot root password in CentOS 6.1/6.2/6.3/6.4/6.5

To reset forgot root password in CentOS 6.0/6.1/6.2/6.3/6.4/6.5 and Fedora 16/17/18/19/20 follow the given below steps:
Step1: Boot your CentOS linux system and press esc key when you see the message “Press any key to enter the menu”

Message on system booting

Step 2: Now, select your operating system if it is dual/multiple boot and pres ‘a‘ to modify kernel argument. If you cancel press ‘ESC‘ key.

Boot menu[

[ads]
Step 3:  In kernel configuration, we have 2 methods to change to single mode. 1 we can use a number “1″, 2 we can use character “S”. Enter after done.

Opened 1 at the GRUB

Step 4:  After reboot type ‘passwd‘ command without username and press ‘Enter‘ key in command prompt.

Change root password

Step 5: Reboot your system and login using the new password.
Reference: http://wiki.centos.org/TipsAndTricks/ResetRootPasswor