As you may know, Microsoft makes ISOs available for both the shipping version of Windows 10 and for various Windows 10 Insider Preview builds. This week, the software giant made a recent release, build 16251, available in ISO form.

An ISO is an image file that represents an optical disc. In Windows 10, you can access these files just like physical optical discs (DVD, typically, but also CD, Blu-Ray and so on): Just double-click the file to “mount” it as a disc in the file system and access it normally. Later, you can virtually remove (or “unmount”) the disc by right-clicking it in File Explorer and choosing “Eject.”

For some inexplicable reason, Microsoft only makes the newest version of Windows 10 available as an ISO download. So if you visit the Windows 10 Download website today, you’ll find that the version you’re getting is 1703, the Creators Update. If you need a previous version, you’re out of luck. (To gain access to the ISO, you need to download and run the Media Creation Tool.)

Over on the Windows Insider website, however, Microsoft makes a few different builds available for download in ISO form. These pre-release Windows 10 builds are rarely up-to-date with the latest build that is available to Insiders, and that’s still true today: Build 16251 was released to Fast ring members a few weeks ago. And since then, Microsoft has also shipped build 16257.

Anyway, an ISO is the quickest route to a clean install, and if you do clean install build 16251, you can quickly upgrade that to the latest Insider build—or, soon, the final Fall Creators Update build—using Windows Update. (You need to add that PC to the Insider program first.)

To get Windows 10 Insider Preview build 16251, just visit the Windows Insider website and select that build from the “Select edition” drop-down. Then, you can choose your language and whether you need the 32-bit or 64-bit version. You can also use the Advanced Options link to choose between different product editions, including Home/Pro, Enterprise, Education, Home Single Language, and China. There are also additional Insider Preview downloads there for those with more complex needs.