You will see many different sources telling you to use a CD, DVD, or even a USB drive. But forget all that, I am going to tell you the easiest and fastest way to install the Windows 10 Technical Preview (or any ISO) from within your current Windows 8.1 installation. It is slightly different if you have Windows 8 or Windows 7, but we'll get into that shortly.
Step 1. Get the Windows 10 ISO file. If you are looking for a way to install it, I am assuming you already have the Technical Preview downloaded. If not, go join the Insider Program here.
Find the ISO file in the explorer, and double click it. This will mount it as a CD drive. If double-clicking doesn’t work, try right click > mount.
Unless it opens up the installation guide (due to the autorun that doesn't normally get triggered by mounting), go to 'My Computer' and there should be a new mounted drive. Double-click this.
It will open a new explorer window with the contents of the ISO. Double-click the setup.exe file, and it will start installing the Technical Preview.
Now, you will see a blue box asking you what you want to keep on your PC (may not let you keep things for Windows 7) and that you do, indeed, want to install the preview. Just follow the steps (it's identical to the installation of Windows 8). It is automated and will tell you exactly what you have to enter (time zone, account, stuff like that), but it will restart your computer a few times.
If you are dual booting, make sure that the first option is Windows. Otherwise, make sure you are paying attention so you can switch to the Windows option in Grub (or whatever boot loader you are using) during each restart. If you are booting multiple Windows, I am assuming that you will want to want to select the boot option that you started this process on.
This is the process I personally used to install the Technical Preview and can attest to it. If you have any questions, or problems, I'll be in the comment section!
The problem with Windows 7 is that it doesn't come with a built-in method to mount ISO files. But, Microsoft has not forsaken you. There is still hope. You can install a program for this functionality here. The link takes you to a Microsoft download page, with instructions how to install "Virtual CD-ROM Control Panel."
Now, just follow the steps I listed above for Windows 8. I would like to warn you that I have not used this method in a long, long time. I highly suggest that you back everything up as any installation can go wrong. I will not be guaranteeing anything, but if you do have problems, I'll be in the comments to help out.
OSX or Linux
In a mixture of sadness and obviousness, you can't even download the ISO from Microsoft's site if you're on either OSX or Linux. However, if you are on any of these operating systems, you probably know how to convince your browser to tell Microsoft that you are on Windows or get the ISO in other forms that I won't get into. Once you have the ISO, you probably want to stick with the standard way of burning a DVD. But, this is all just unofficial conjecture.
If, for some unknown reason, this did not work for you -- and if you couldn’t find help in the comments -- there are alternatives. The easiest of them perhaps is using a bootable USB method (click here for a how-to).
Also, If you are not sure that you want to actually install the technical preview, there are other ways to test the OS! You can make a virtual machine on your computer that will not endanger your files/programs (click here for a how-to).