Apple: Reset User Password in Mac OS X

There are two ways to reset the user password in Mac OS X:

<< On Mac OS X >>
If you boot in your system, you can change the password. It's used to change it if you didn't forget it.

Boot your Mac and open System Preferences > Users and Groups.
You can see the users. Press the user what you want to change the password and select Change Password. You will be asked for your password.


<< On Recovery >>
When you forgot the password and you can't use OS X, you have to use the new Recovery.

To boot in Recovery system, press Command and R keys in boot and hold the keys until you see the Apple icon. If you have a Mac with Internet Recovery, read > http://support.apple.com/kb/HT4718. If your Mac has got a wireless keyboard, hold them when you hear the startup sound
When it starts, select Utilities > Terminal, and type:
resetpassword

Press your user and type your password. Finally, reboot.

This doesn't work for FileVault.


Reference:
Reset the user password in OS X Lion, Mountain Lion and Mavericks
https://discussions.apple.com/docs/DOC-4101