UPDATE: If you read the comments, someone posted a torrent of the Windows RT 8.1 ISO with the Surface drivers already inlines. Check the comments for the Magnet link, and skip steps 2, 3, and 14.
I upgraded my Surface RT to Windows RT 8.1 RTM tonight. It's not as easy as a regular Windows 8.1 upgrade, and it has some risk associated with it. You could definitely brick your Surface if you do not do this properly. I am not responsible for bricked devices.
That said, here's how I did it: NOTE: You will need to use a separate Windows 8 / 8.1 / Windows Server 2012 / R2 system to do some of this work on, in addition to your Surface. There will be some work required in order to integrate the drivers and grab your Product Key.
ALSO NOTE: None of this is illegal because you will use your existing and legitimate Windows RT 8.0 Product Key to activate your copy of Windows. The key I provide for installation is just to get you through the install process. It *will not* activate.
Step 1 (Work machine): Download the RTM package from BitTorrent. Here is the MAGNET link. I've been downloading WZOR ISOs for almost a decade. They are always legit and never messed-with. You can trust them. NOTE: The Surface drivers the post mentions are already in the torrent.
Step 2 (Work machine): Extract the Surface Drivers to an easily-accessible folder.
Step 3 (Work machine): Create a folder on your C drive called "test".
Step 4 (Work machine): Double-click the ISO from the torrent to mount it to your system.
Step 5 (Surface): Clean a 4 GB or larger USB key, and plug it into your Surface.
Step 6 (Surface): Hit the Start button, type "regedit", and hit Enter.
Step 7 (Surface): Navigate to HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion. Then go to File | Export. Type the name "registration" in the file name box, and save it to your Documents folder.
Step 8 (Surface): The only keys you need are "DigitalProductId" and "DigitalProductId4". Select those in the text editor, then hit Ctrl+X. Hit Enter after the 3rd line at the top of the file, then hit Ctrl+V to paste the keys. Then, select everything from the top of the file to the end of the "DigitalProductId4" key, then hit Ctrl+x again. Then hit Ctrl+A to select all, and hit Ctrl+V again to paste only what we need. Trust me, this method saves several minutes of scrolling to delete the rest of the exported data.
Step 9 (Surface): Copy the file to your USB stick and plug it into your Work machine.
Step 10 (Work machine): Repeat steps 6 through 8. This creates a backup of yoru regular product key so your "work" machine registration stays intact.
Step 11 (Work machine): Double-click the registration.reg file on the USB key to import your Surface RT product key onto your work machine.
Step 12 (Work machine): Download Ultimate PID Checker, extract the RAR file, and run the application. In the screenshot in the link, you'll see a Windows icon button next to the "Go" button. Click it, and take down the Installation Key. You'll need that later to legitimately activate your copy of Windows RT 8.1.
Step 13 (Work machine): Copy the files from the mounted ISO to the USB stick.
Step 14 (Work machine): Inline the Surface drivers with both the boot.win and install.wim files by executing the following commands, one at a time. Substitute X for the drive letter of your USB stick, and [DRIVERSPATH] with the path you extracted the Surface RT 8.1 drivers to, when necessary. NOTE: Put quotes around "[DRIVERSPATH]" as the extracted path has spaces in it.
- Dism /Mount-Image /ImageFile:X:\sources\boot.wim /Index:2 /MountDir:C:\test\
- Dism /Image:C:\test\ /Add-Driver /Driver:"[DRIVERSPATH]"
- Dism /Unmount-Image /MountDir:C:\test\ /Commit
- Dism /Mount-Image /ImageFile:X:\sources\install.wim /Index:1 /MountDir:C:\test\
- Dism /Image:C:\test\ /Add-Driver /Driver:"[DRIVERSPATH]"
- Dism /Unmount-Image /MountDir:C:\test\ /Commit
Step 15 (Surface): Remove the USB stick from your Work machine and plug it into the surface. Navigate to the drive and double click the setup file.
Step 16 (Surface): When it prompts you to download updates for Setup, SKIP THIS PART. Failure to do so will not let you use the key for the next step.
Step 17 (Surface): Enter the following Product Key: NK2V7-9DWXG-KMTWQ-K9H9M-6VHPJ
Step 18 (Surface): Select "Keep my personal files". If you are on the 8.1 Preview, this will also keep most (if not all) of your settings.
Step 19 (Surface): Complete the installation. It will take about 30 minutes, with at least 6 reboots.
Step 20 (Surface): Select "Customize" for the installation settings. It will walk you through your options, then let you log in under your main account and re-download your settings.
Step 21 (Surface): Hit WinKey + C, select "Settings", then "PC Settings" at the bottom. Then select "Activate Windows". In the space provided, type in the Product Key from Ultimate PID Checker. After 30-90 seconds, your copy should be activated.
Step 22 (Work machine): Navigate to your documents folder, and double click "registration.reg" to return your registry settings back to its previous state.
Step 23 (Work machine-optional): Close and re-open Ultimate PID Checker, hit the Windows Key again, and verify that your Windows Key does not say "Windows CoreARM".
Step 24: Enjoy Windows RT 8.1 RTM on your Surface RT.