If you are in the Windows Insider Program (you really should be! More info here) then you can switch from a regular Windows 7, 8, 8.1 or 10 installation to a Windows 10 Insider Preview installation.
But if you do not want to go that way or if you want to test Deployment for instance 😉 then you need an ISO for that. And the good news is Microsoft provides these. Ok not for all the Builds, but they appear regularly!
The latest Build available is Build #18290. This build is from December, so you are pretty close to the latetst one.
Get your Windows 10 Insider Preview Build now:
Here you can choose which version you would like:
And the language: