How to fix a Windows 8.1 upgrade that ends with a black screen
If you see "The parameter is incorrect", or "The set command specified is not valid", or "...is not recognized as an internal or external command...", or a similar message, check Step 3 again and make sure you executed the command properly. If you get other errors about not finding the BCD, then you might be booting into Legacy Mode, make sure your using UEFI.
Important: Windows will continue to start in Safe Mode every time you reboot unless you undo what you did in Step 3.
bcdedit /deletevalue safeboot