Use a Kali Linux distro on a usb
There are several ways of doing this, as man wuzi said, you can use a kali linux distro on a usb and use its many offline cracking tools (I believe OPHCrack is on there, that's what i would use personally) that would be by far the easiest, or you could get a CMD window open before logon (again, several ways of accomplishing this) and you should have administrator privileges and could possibly change the password without cracking it.
This should take only 10 minutes or less.
#> net user <username> *
where <username> is the name of the account you want to change. If you want a list of users for the computer, type: net user.
This will ask you for a new password, Just type a new password or leave it blank, confirm again with the same password and you are good to go.
Click on my name use my guide. It works on 8.1 as well
Reset using built-in options in Win-8.1 (takes 5-min) Advanced Re-boot (Reset only password) (I have no idea why is it built-in)
or u could just get konboot.. does it the EFI way, watch out on mac with it.. i got errors due to connected icloudaccount! (did the trick though, just gave some errors to the address list and others)
NT Password and Ophcrack are the free tools you can use to reset windows login password.
NT Password and Ophcrack works fine for me