How to Reset any user Password On Ubuntu 14.04 LTS
Step 1 - Restart the PC and just while booting the OS press 'ESC' button twice.
Step 2 - You will see the window of these beloow options.
Advanced options for Ubuntu
select the -->'Advanced options for Ubuntu' and press enter
Step 3 - You will see another window and Select --> Ubuntu, with Linux 188.8.131.52-generic(recovery mode) .
Step 4 - After that select the root option (Drop to root shell prompt)
Step 5 - Execute below command
--> mount -rw -o remount /
After successfull it won't show any error it will just execute the command.
--> passwd user_name_which_you_want_to_reset
After that it will ask you to enter New password and re-enter the new password.