Microsoft have made available the latest Windows 10 Insider Preview build to download as an ISO. Build 20150 was released yesterday to the Fast Ring (soon to be known as the Dev Channel) and it can be downloaded as an ISO. The ISO is for Windows Insiders to perform a clean install of the OS. Normally the ISO version is a few weeks behind the latest insider build but this time they have been released simultaneously.
Build 20150 switched back to the rs_prerelease branch which has new WSL features and a lot of fixes.