The following works for Winfows 8.1 and Windows 10.
1. You need NOT to be logged in with the same user account you want to rename.
2. Open Control Panel->Users and change your account name.
3. Run mmc.exe with administrative rights, open Console Root->Computer Management->System Tools->Local Users and Groups->Users, right click on user and select Rename option.
4. Run regedit.exe with administrative rights, open HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\ProfileList, find your profile’s folder and change the value for the key ProfileImagePath.
5. Rename user folder. C:\Users\oldusername to C:\Users\newusername
If you cannot rename the folder and you get an error indicated that folder is in user try the following:
5.1. Usually, what prevents you from renaming a folder(if not a malware) is the wmpnetwk.exe process. Run services.msc, stop and DISABLE(yes! disable it!) service Windows Media Player Network Sharing Service. Try again to rename the user folder.