Other Reasons for Doing this:

• Reset a Mac OS-X  Password
• Computer Won’t Let me Log in After Installing Yosemite
• Can’t login, password is wrong but I know it isn’t
• Yosemite is asking me to login with my Apple ID / iCloud password, I have even reset my password on appleid.apple.com and it still won’t let me login
• I lost or forgot my password

OS:

• Mavericks
• Mountain Lion
• Yosemite

Solution:

1. Boot into recovery mode by
   • Shut down your Mac
   • Switch on your Mac and hold down Command + R (hold down both keys at the same time, keep holding them down)
   • When you see the Apple logo and loading bar appear, you can let go of Command + R
2. Let it finish loading and you should now be in the recovery mode utilities screen, if not repeat the above.
3. In the menu at the top start Terminal
4. Now in the black Terminal window, type resetpassword and hit Enter (return)
5. You should now be prompted to reset your password, enter your new password
6. In the menu top left select restart to restart you computer and boot into Mac OSX
7. Log in using your new password