This is not the only method but this worked for me:
Boot from a Windows 8 installer and invoker a command prompt. Determine the drive letter of the Windows 8 system that you want to change the password of. Let's use D: as an example:
1. Temporarily copy D:\windows\system32\utilman.exe somewhere.
2. Rename D:\windows\system32\cmd.exe to D:\windows\system32\utilman.exe
3. Reboot
utilman.exe is the program that is invoked when you click that accessibility icon in the bottom left corner of the login screen. By renaming cmd.exe to utilman.exe, when that icon is clicked, a command prompt invokes instead. From there any local password can be changed (admin rights required?).
net user <someone> *
William actually posted this solution in a very similar question