1. Add the English US language:
- Go to the Control panel and select “Add language”
- Add English => English US
- Make sure that it’s downloaded and installed, just adding isn’t enough
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