Windows - To reset the password on your Windows 2012 server, simply complete the following steps:

To reset the password on your Windows 2012 server, simply complete the following steps:

• Boot from the Micrsoft Windows Server 2012 DVD
• From the Windows Setup menu, click “Next”.
• Select “Repair your computer”
• Under Choose and option, click on “Troubleshoot”.
• Under Advanced options, click “Command Prompt”.
• At the command prompt, run the following commands: d: cd windows\system32 ren Utilman.exe Utilman.exe.old copy cmd.exe Utilman.exe
• Close the command prompt and then click “Continue”.
• The server should now boot and present the logon screen. Here click Windows Key + U.
• At the prompt you can now change the password, by typing the following command: net user administrator Password123 This will set the password for the Administrator user to be Password123 (case sensitive).

Refer below link.
http://www.kieranlane.com/2013/09/18/resetting-administrator-password-windows-2012/