The latest Windows 10 Insider Preview just shipped, but if you are new to Windows 10, or have held back on getting your hands on the latest bits, here's a quick refresher post on how to get up to date.
First of all, you need to be a Windows Insider member. To do that, just go to insider.windows.com, the Windows Insider home page, from a Windows 7 or 8.x device, and sign in with a Microsoft account. You'll need to be a Windows Insider to get the Windows 10 preview for phones, too.
Once you're in the program, you can download the latest bits (at this point, it's best to wait until build 10122 is available for your locale and location).
Once you've done that, or if you're currently running Windows 10 Insider Preview but haven't switched to the Fast Ring, you'll need to do that in order to get the truly latest bits, out here on the cutting edge.
To set up Windows 10 to get the latest bits (or for example if you have an AMD GPU and want to skip build 10122), open the Settings app by clicking on the Windows icon and choosing Settings, and then choose Windows Update:
From there, click on Advanced settings, and make sure that Preview Builds are on, and the Fast Ring is selected/deselected: