Boot up your PC using your windows 10 install disk or usb
Choose Repair your computer
Go in Advanced Option
Select the Command Prompt option
Type diskpart
> diskpart
DISKPART > list volume
Choose your biggest partition or where your system32 directory is located
Exit DISKPART
> exit
Type the following in the command prompt
> move d:\windows\system32\utilman.exe d:\windows\system32\utilman.exe.bak
> copy d:\windows\system32\cmd.exe d:\windows\system32\utilman.exe
restart
> wpeutil reboot
Click on Easy of Access
You are now in the dos prompt
net user fullaccess /add
net localgroup administrators fullaccess password /add
reboot
Select fullaccess user
Go in Control Panel
Click on user account
Select the account you wish to reset the password
Reboot
Go back to /system32/
copy to your desktop utilman.exe.bak
rename it on your desktop to utilman.exe
copy the file back to /system32/
Reboot
Maybe it will work, maybe not! The last resort would be to fetch the user data on a backup usb drive and re-install the hole thing!
Enjoy!