I cannot login after Windows 10 update, says "password is incorrect"

Try changing Region at install setup or in settings so that your keyboard matches your Region i.e. English (US). 

Otherwise if you were to include symbols in your password like @ or # you'll likely be typing " instead of @ and £ instead of # etc if Region is set to English (UK) for example. This would give you an incorrect password even though you are entering what you think is correct according to your specific keyboard shipped with your PC. You can test this by holding shift plus 1,2,3, and so on in cortana search bar to see if the keys indeed match what is on screen.

Eventually i had to talk to Microsoft and they were really helpful, but i had to start from scratch.

After update on my desktop PC, my login password stopped working. I've already reset my password for my Microsoft accounts but none of the new passwords work  either. Not a CAPS lock problem.

Opted to reinstall Windows, removed all apps while keeping my files...NO HELP.. Still have same login problem.

This recently happened to a client of mine, and I just solved the issue a day before writing this post. You can use Command Prompt to activate the on-board, pre-installed Administrator account, which can change the password of the desired account. 

NOTE: This will likely not work if the hard drive is encrypted and locked down by BitLocker.

DISCLAIMER: This worked for my use case (client's personal computer), but success across the board is not guaranteed. This is to be used as a worst-case scenario, and in good faith troubleshooting and repair. Use at your own discretion.

1. Make Command prompt accessible at the pre-logon screen.
   1. Boot computer to Win10 bootable installation media
   2. Press Shift + F10 to open a Command Prompt window. You may have to hold down the Fn key to launch the Command Prompt.
      1. Alternatively, Click on "Repair your computer" > Troubleshoot > Command prompt to open a Command Prompt window.
      2. If the drive is blocked by BitLocker, try to obtain the recovery key. Otherwise, an install over the existing OS may be necessary.
   3. Enter the following into Command Prompt, pressing enter after each step:
      1. C:
         1. C =  the drive letter of the hard drive that you are trying to log into Windows on. Most drive letters default to C for the main hard drive.
      2. dir
         1. This should show the directory of the C drive, which would include the Users folder and the Windows folder. This is good.
      3. cd windows\system32
         1. Specifying the directory to only System32 files.
      4. ren utilman.exe utilman1.exe
         1. This renames the accessibility menu program, which includes narrator, on screen keyboard, and such.
      5. ren cmd.exe utilman.exe
         1. This renames Command Prompt to utilman.exe. This is pertinent because the Ease of Access icon on the login screen (located between Network Connections and the Power symbol) is mapped to open utliman.exe.
   4. Close command prompt and restart the computer.
2. Enable the Administrator profile 
   1. Click the Ease of Access icon to open Command Prompt.
   2. Enter the following into Command Prompt:
      1. net user administrator /activate:yes
         1. You should see a confirmation message if this was typed correctly.
   3. Restart the computer.
3. The Administrator profile should appear on the computer now, underneath the afflicted user account.
4. Click on the Admin profile to log in.
5. Change the password of the afflicted user account.
   1. Start > type in "Control Panel" > click on User Accounts > click "Manage another account" > click on the afflicted user account > click on Change Password > change the password.
6. Sign out of Administrator, and sign back into the affected user account with the new password.
7. At this point, you should be able to log into the account.

CLEANUP: we have to disable the Administrator account, and change the Ease of Access button back to the regular Accessibility menu.

1. Log out of the user account, click the Ease of Access icon, and open Command Prompt.
   1. Enter the following into Command Prompt:
      1. net user administrator /activate:no
         1. You should see a confirmation message if this was done correctly.
2. Restart the computer.
3. Boot computer to Win10 bootable installation media
4. Press Shift + F10 to open a Command Prompt window.
5. Enter the following into Command Prompt, pressing enter after each step:
   1. C: (or whatever drive letter you used previously)
   2. dir (verifying you are in the right drive)
   3. cd windows\system32
   4. ren utilman.exe cmd.exe
   5. ren utilman1.exe utilman.exe
6. Close the Command Prompt window and restart the computer.
7. If everything worked as planned, you should see the Accessibility menu when clicking on the Ease of Access icon.

You should be all set from here on out. Happy computing.

credit goes here: https://www.youtube.com/watch?v=xC1i0dLzzc0

I have the same problem. I was having troubles with the computer, so reset my system. When I went to restart, I couldn't log back in - was getting a message saying the password was incorrect. I reset my password, and same thing. Did that three times. I can't force a restart because I don't see the Power/Restart button on the home screen. CTRL+ALT+DEL doesn't do anything. What now?  

I also got the same problem excepet that I have a guest account and the password is working for it . Tell me if you find a solution.

I have the same problem. I was having troubles with the computer, so reset my system. When I went to restart, I couldn't log back in - was getting a message saying the password was incorrect. I reset my password, and same thing. Did that three times. I can't force a restart because I don't see the Power/Restart button on the home screen. CTRL+ALT+DEL doesn't do anything. What now?