Forgotten your password?

Turn your computer on.

Press ESC at the GRUB prompt.

Press e for edit.

Highlight the line that begins kernel ......... or linux

Go to the very end of the line and add rw init=/bin/bash

Press F10 to boot your system. Your system will boot up to a passwordless root shell.1

Now issue the command passwd username *where "username" is the user for which you want to change the password.

Then you will be asked to give new password:

Enter new UNIX password:

Reboot