Windows 10 Insider Preview is available for download on Microsoft.com.
Here you can download the 64 bit version (3.78 GB) and the 32 bit version (2.77 GB).
How to install it?
You will need to have Windows 7, 8 or 8.1 on your computer. If you have Windows 7, you will need Daemon Tools or a similar software that can mount .iso files.
If you have Windows 8 or 8.1, you won’t need any virtual disk emulator software since Windows 8 and 8.1 have an already built in software for this.
So mount the .iso file and click on “setup”.
Be careful, the new Windows 10 will erase your current Windows version from where you are installing it.
If you have Windows 7 for example and also an Windows 8, on the same computer, you can make the update from Windows 8. You will still be able to access your old Windows 7 after you make the update from Windows 8 to Windows 10 Insider Preview.
The installation for the new operating system will take a while, around 30 minutes at least.