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Microsoft released Windows 10 IoT Core Insider Preview today.

Download Windows 10 IoT Core Insider Preview with support for Raspberry Pi 2
Unzip Windows_IoT_Core_RPI2_BUILD.zip
Run Command Prompt as Administrator, navigate to the unzipped folder.

Insert SD card into card reader.
Install image file (flash.ffu) to your SD card.

dism.exe /Apply-Image /ImageFile:flash.ffu /ApplyDrive:\\.\PhysicalDriveN /SkipPlatformCheck

If your SD card is disk number 3, use /ApplyDrive:\\.\PhysicalDrive3
To find the disk number of your SD card:

diskpart
list disk
exit

Remove your SD card with Safely Remove Hardware. Your SD card is now ready to boot Windows 10. Insert SD card into your Pi, Connect HDMI monitor, network cable, power supply. Boot Windows 10 IoT Core Insider Preview.

Reference
PowerShell documentation