Microsoft wants you to have a free Windows 10 upgrade, and they really want you to use Windows Update to get it over the Internet. If you have a PC that qualifies for the upgrade, you should do this if possible. It really is the best, fastest, cleanest option for the vast majority of Windows PCs.
But a lot of my readers want traditional installer files, either on a DVD or in an ISO image file that is the equivalent of a DVD. If you are old school, you want an ISO because installing Windows on bare metal is just what one does, period.
I get that, I really do.
Having that installer image available gives you access to some installation and repair options that you don't have otherwise, such as the option to create bootable media. That's mandatory if you want to set up a dual-boot installation, for example.
For the Insider Preview, Microsoft makes an ISO available for download with every Slow ring release. But Fast ring updates are delivered only via Windows Update, and at this point we don't know how Microsoft will make ISO copies of the final Windows 10 release available for download.
I know long-time Windows users have wake-up-in-a-cold-sweat nightmares about Windows upgrades gone horribly wrong. But those days are long gone. I have done hundreds of clean installs and upgrades, on physical and virtual machines, using the Windows 8.1/10 installer family.
The staged installer used with Windows 8.1 and Windows 10 is as close to failsafe as software gets. If something goes wrong, you can roll back easily. (I've done those rollbacks a lot during this beta.)
After the upgrade is complete, you can create an ISO file from the cached upgrade files. That ISO file can be used to install Windows on any PC, provided you have the right product key (more on that in a minute). I've just done this with Build 10158.
Or you can start the upgrade, wait until the installer files are fully downloaded, and then cancel. In either case, you'll find the large installer file stashed in a hidden folder called C:\$Windows.~BT, in a subfolder called Sources.
Here's how to find it in File Explorer. (Note that I've enabled the option to view hidden files.)
This file, Install.esd, is saved using a compressed format introduced with Windows 8.1 and specifically designed for electronic software distribution (thus the .ESD file extension).
If you're an IT pro responsible for deploying Windows in your organization, you can use the Windows Assessment and Deployment Kit (ADK) to decompress and decrypt that image, turning it into an ISO file using Microsoft-approved tools. A blog post by Microsoft MVP Johan Arwidmark contains detailed instructions on this technique.
If you don't want to install the full deployment toolkit, here's a faster way to build that ISO using the same tools, packaged by community members for this specific purpose.
Step 1: Download the ESD-Decrypter files. There's a download link in this post on the Microsoft Answers forum, although you can also find the files elsewhere. Note that this file is saved in 7z format, so you will need a third-party decompression utility such as 7-Zip to extract it.
Step 2: Extract the ESD-Decrypter files to their own folder and then copy the Install.esd file to the same folder.
Step 3: Right-click the Decrypt command file and choose Run As Administrator. (It's a batch file, so you can see exactly what it's going to do.)
Step 4: Choose the first option in this menu and press Enter.
The command file displays the status of current operations as it works. After a few minutes (the exact time depends on your hardware), you end with an ISO file in the same folder as the ESD.
You can now create bootable media from that ISO file. You can also attach it to a virtual machine to install Windows 10 in a VM. You can double-click to mount it in File Explorer in Windows 8.1 or Windows 10 and run Setup directly from within Windows.
To install Windows 10 Insider Preview build 10158, you need to enter a product key at the start of installation. You don't need a unique product key. Instead, use the correct global key for your edition.
For this build, there's a separate global key for each of the three editions:
Windows 10 Home Insider Preview build 10158 product key:
Windows 10 Pro Insider Preview build 10158 product key:
Windows 10 Enterprise Insider Preview build 10158 product key:
Note that these keys should work for build 10159 as well.
This is the first public release for which you must enter a product key for installation. The key numbers are no longer embedded in the build, and there's no longer a Skip option during a setup from bootable media.