click power buttonthat is on the extreme right corner.
click restart by holding Shift key continuously until Recovery Menu gets popped up.
select Advanced options(Recovery Mode) to set the password for the user.
selectrootin the current pop up window
A interactive shell starts prompting which asks for a password ,just Click Enter .
enter the following commands to reset the password of a log in user.
mount -n -o remount,rw /
passwd username
enter the new-password :
re-type the new password :
reboot
Now you could log in with the new password that you had set.