1. Boot the system and when you see the following message "Press any key to enter the menu",press any key. (You will see the list of available kernel versions.)
2. Press e in order to edit commands before booting.
3. Highlight the list item with vmlinuz in it by using the arrow keys and press e.
4. Now type single or init 1 at the end of the line.
5. Then press enter and b to boot the system with the new argument. (The system will boot into single user mode and you will see bash prompt)
Now it's time to change the password:
6. Type passwd
7. Type reboot to restart the system.