Download both the 8.0 and 8.1 installers
Launch the 8.0 installer [Windows8-Setup.exe]
Enter your key for 8.0 and begin downloading
Immediately pause your download at 1% (don’t quit at the estimating time screen), then exit
Launch your 8.1 installer [WindowsSetupBox.exe]
You’ll get an error saying element can’t be found or something like that, you’re ok – just close out
You’re downloading 8.1
Create a Notepad text file and copy the following lines:
In this case because my installation is for Windows 8.1 Pro edition, I’m specifying “Professional” in the EditionID, for the core edition use the EdtionID Core.
Finally, save the text file as ei.cfg inside the SOURCES folder from the bootable drive you just created with the Windows 8.1 installation files.
This method will basically allow you to install Windows 8.1 Pro or Core edition as a trial for about 30 days without entering a Windows 8 product key during the installation — because, once again, it won’t work –.
But of course you have a valid retail key, so after everything is installed and configured, you can go to PC settings, PC & Devices, PC info, and here you can change and activate your product key.
I used This source.