Update stock Microsoft Surface RT (2012) to Windows RT 8.1
This guide is a WIP.
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.
The 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.