How can I change username in Windows 8.1?

As far as I know, you can rename your user in Win8.1, but the directory your profile lies in won't change to reflect the new user name! It will always stay the same as it was created.

Let's assume you named your user correctly ('John') in the first place, then handed your computer over to a friend named 'Heather' and she changes her profile name accordingly. From now on Windows will welcome her as 'Heather', but her user files will remain in the profile directory C:\Users\John!

The only way to circumvent this is to create a new user, transfer all files from the old user profile to the new one and then delete the old user profile.

If you want to change your default user folder, this article will help you.
Let me bring you short version of that article:

1. Back up to recover if smtg goes wrong
2. Log in as another (administrator) user. Don't try to do it while you are logged in that user
3. Move/rename C:\Users\John's PC to C:\Users\John
4. Open Regedit.exe (Win+R -> tupe regedit.exe -> hit enter) and go to this path: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\ProfileList
5. Check all subkeys until you find your user path (C:\Users\John's PC) like this:
6. Double click on ProfileImagePath and change value to your new folder path (in your case it's C:\Users\John).
7. Click OK, close all, reboot (or log out/switch user) and log in to that user. Now user profile must be in C:\Users\John. will help.

Warning: After this some programs may not work properly, but reinstalling them will fix the problem.

I don't recomend you do this (especially if you are not advanced user).
The safest way is to create a new user, manually copy your Desktop (C:\Users\PreviousUser\Desktop) and Documents to your new user.