Update stock Microsoft Surface RT (2012) to Windows RT 8.1

I have a 2012 Microsoft Surface RT that was never updated -- no 8.1, no firmware. However, I can't update it normally due to it being unable to access Windows Update or the Store. So, follow these steps:

The Problem

The preinstalled version of Windows RT 8 cannot access Windows Update or the Windows Store. So, Windows RT 8.1 installation media must be used. But, the installation media wants the firmware to be updated, and the firmware packages do not seem to exist anywhere online other than Windows Update. Also, while I can get a Windows RT 8.1 recovery image for the Surface RT from Microsoft's website, restoring the image immediately bombs out.

Windows RT 8.1 Setup Workaround

Edit the compatibility database to skip the check that is being triggered.

Edit /sources/drvmainarm.sdb with a hex editor and replace 7B 00 39 00 33 with 7B 00 36 00 33. This changes the start of the GUID for the failing check, which allows the Windows 8.1 Setup program to continue.

Afterwards

Follow one of these guides:

• https://www.reddit.com/r/WindowsRT/comments/p6jtyk/getting_windows_rt_81_to_download_updates_from_a/
• https://surfacetip.com/install-windows-rt-8-1-update-3-2017/

For me, the second guide was the one I needed.