Prerequisites

A Windows ISO

There's plenty of other places you can find one but personally I would do one of the following.

  1. If you have a Microsoft account that's enrolled in the insiders program you can download an insider preview ISO.
  2. If you just want a stable current version of you can download the media creation tool for Windows 10 and use it to generate an ISO.

Installing the new version

With the ISO downloaded let's start by mounting it, you could just right-click and choose Mount or you could use a powershell command instead.

Mount-DiskImage -ImagePath "c:\Windows-Insider.iso"

Now the ISO is mounted let's have a quick look at my filesystem looks.

Get-PSDrive -PSProvider FileSystem

You can see I've got Windows installed on the C: drive and F: is the mounted Windows install ISO. D: is where I'm going to install my second version of Windows and E: is a spare.

We're going to use the Deployment Image Servicing and Management (dism ) to do this and the first job we'll use it for is finding out which images are available in the ISO using the Get-WimInfo command. Depending how that ISO was created that info is stored either in a .wim file or a .esd file.

If you used option 1. above it will be in a install.wim file so we use the comman

 # List available editions for .wim based ISO
 dism /Get-WimInfo /WimFile:F:\Sources\install.wim

If you used option 2. above it will be in an install.esd file so we use the command:

 # List available editions for .esd based ISO
 dism /Get-WimInfo /WimFile:F:\Sources\install.esd

This will print a list of all the editions of windows available within the ISO and the index of each, here's a snippet of the output I get.

I'm going to install the image at Index 6 (Windows 11 Pro) from the F drive to the D drive.

dism /Apply-Image /ImageFile:F:\Sources\install.wim /Index:6 /ApplyDir:D:\

This will take a while, but you should see a nice progress bar like this:

Once complete we just need to add the new installation to the boot menu.

bcdboot D:\Windows

And, optionally, set its description. 

bcdedit /set {default} description "Windows 11 Insider Preview"

Now when you reboot, you'll get the choice of which Windows version to boot.