The following Windows 8 / Windows 8.1 password reset trick only works if you are using a local account. If you use an email address to log-on to Windows 8 then you should follow This Link
Procedure:
1. Access Advanced Startup Options.
2. Touch / click Troubleshoot, then Advanced Options-> Command Prompt.
3. At the Command Prompt, type the following and press Enter
copy c:\windows\system32\utilman.exe c:\
You should see a 1 file(s) copied confirmation.
4. Next, type this command, and press Enter:
copy c:\windows\system32\cmd.exe c:\windows\system32\utilman.exe
Answer with Y or Yes to question about the overwrite of the utilman.exe file. You should again see 1 file(s) copied confirmation.
5. Remove any flash drives or discs that you may have booted from in Step 1 and then restart your computer.
6. Once the Windows 8 log on screen appears, click Ease of Access icon at the bottom-left corner of the screen. Command Prompt should now open.
With the command prompt window open, you can reset your Windows 8 password as follows.
7. Type the following and press Enter (replace username with your user name, and password with your desired new password)
net user username password
For example, on my computer, I need to execute the command
net user "San User" abcd1234
(You only need to use double quotes around your username if it happens to have a space in it)
If you get the error message "The user name could not be found", execute net user to see the list of Windows 8 users on the computer for reference and then try again with a valid username. A System error 8646 / The system is not authoritative for the specified account... message indicates that you're using a Microsoft account to login to Windows 8, and not a local account.
8. Close Command Prompt.
9. Login with the new password you set in Step 7.
10. Now that your Windows 8 password has been reset and you're back in, either create a Windows 8 password reset disk or switch your local account to a Microsoft account.
11. Finally, you should reverse the hack that makes this password reset trick work in Windows 8. To do that, repeat Steps 1 & 2 above.
Once Command Prompt window is open again, execute this command:
copy c:\utilman.exe c:\windows\system32\utilman.exe
Confirm the overwriting by answering Yes and then restart your computer.
4. Next, type this command, and press Enter:
copy c:\windows\system32\cmd.exe c:\windows\system32\utilman.exe
Answer with Y or Yes to question about the overwrite of the utilman.exe file. You should again see 1 file(s) copied confirmation.
5. Remove any flash drives or discs that you may have booted from in Step 1 and then restart your computer.
6. Once the Windows 8 log on screen appears, click Ease of Access icon at the bottom-left corner of the screen. Command Prompt should now open.
With the command prompt window open, you can reset your Windows 8 password as follows.
7. Type the following and press Enter (replace username with your user name, and password with your desired new password)
net user username password
For example, on my computer, I need to execute the command
net user "San User" abcd1234
(You only need to use double quotes around your username if it happens to have a space in it)
If you get the error message "The user name could not be found", execute net user to see the list of Windows 8 users on the computer for reference and then try again with a valid username. A System error 8646 / The system is not authoritative for the specified account... message indicates that you're using a Microsoft account to login to Windows 8, and not a local account.
8. Close Command Prompt.
9. Login with the new password you set in Step 7.
10. Now that your Windows 8 password has been reset and you're back in, either create a Windows 8 password reset disk or switch your local account to a Microsoft account.
11. Finally, you should reverse the hack that makes this password reset trick work in Windows 8. To do that, repeat Steps 1 & 2 above.
Once Command Prompt window is open again, execute this command:
copy c:\utilman.exe c:\windows\system32\utilman.exe
Confirm the overwriting by answering Yes and then restart your computer.