In addition to the Windows Insider Program, qualified MSDN subscribers can also grab the Windows 10 Technical Preview ISO files from the MSDN pages, where the New Subscriber downloads page looks like this at the moment:
Chances are high that most readers of this blog will want item 7 counting down from the top: the x64 English version.
For those installing on Windows 8.1, all you really need to do is to copy the ISO to a directory accessible on your system, then double-click that file in Windows File Explorer. It should mount as a CD drive, but if that doesn’t work you can select “Mount” from the pop-up right-click menu to achieve the same result. Then you need only navigate into that drive (or mount point, rather), where you’ll launch setup.exe from its root directory. This will commence the installation, and you’ll be off and running. Just remember: the usual rules on previews apply. You should always make a backup of your old system image before installing a new OS. On the off chance that something in the new install goes amiss, you can point the repair utility at that system image and restore it (which is why I usually copy such things to a fast, USB3 external drive to reduce overall rebuild time should that prove necessary). And remember, unless somebody comes out with a registry hack to cheat the “real Windows 10 installer” when it finally emerges next year, you’ll have to perform a clean install of the final version on the test machine that will be running the preview until that time comes.