How to upgrade a Surface RT to Windows 8.1 Preview with language mismatch

If you get an error "The update is not applicable to your computer", follow these steps.

1. Add the English US language:

2. Set the OS Installation and default to English US in the registry:

Start “regedit”
Navigate to HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Nls\Language
Change values for InstallLangauge and Default to 0409
Reboot the system, After reboot the system should startup in English US with Qwerty keyboard
(can be changed afterwards back to your preferred layout / Language)

3. Check if the language is set to English using DISM:

4. Install the Security Update KB2849636

Downloaded the MSU file to a temporary location (e.g. C:\TEMP8)
Open an elevated Command Prompt
Type: Expand –F:* C:\TEMP8\Windows8-RT-KB2849636-arm.msu C:\TEMP8\
Type: DISM.exe /Online /Add-Package /PackagePath:c:\TEMP8\Windows8-RT-KB2849636-x64.cab

5. Restart the computer, open the Store and click on the Windows 8.1 download.

6. You should see the Installs screen and (slowly) moving progress bar.

7. Enjoy the Windows 8.1 preview