Microsoft plans to release the latest build of Windows 10, presumably Consumer Preview) in the late January 2015. If you are currently evaluating Windows 10 Technical Preview, you will be able to upgrade to the latest build through Windows Update. Here is how.
For those who prefer a clean installation, here are 4 ways to try out.
But for some reason, you just want to jump ahead, upgrading your current Windows 7 or 8.1 computer directly to the latest build of Windows 10, Microsoft has released a free tool that prepares your computer to get ready to be upgraded straight to Windows 10 through Windows Update in early 2015.
Head over to this download page, and hit Prepare this PC now button to download the proper version of preparing tool for your PC.
Double-click the tool you just downloaded and follow the instruction to get your PC prepared.
Note that your PC needs to be either Windows 7 SP1 or Windows 8.1 Update and a language supported in Windows Technical Preview. Or, you will be prompted by a pop-up like this:
I am not sure if downgrading back to Windows 7 or 8.1 will be available. I guess we will see by then. But since moving forward is always what we do, I probably will take my chance and upgrade one of my main machines directly to Windows 10 and see how it works out.