1.start your machine 
2.press E in the time of booting until grub boot loader appear
3.now remove ro quiet splash
4.add rw initrd=/install/initrd.gz init=/bin/bash
5.after editing press ctrl+x
6.wait for terminal when terminal appears clear the terminal 
7.enter whoami
8.you can see you are the root user
9.now you can do any thing
10.you have authority to access any directory any file and have full power of the machine.
11.after work enter exec /sbin/init 
to save all the changes.
12. now you can login with new password.
thats all.