Microsoft is rolling out Windows 10 20H2, its next Windows 10 feature update, to testers in the Release Preview ring today, September 18. Officials said they "believe" test build 19042.508 (KB4571756) is the final build and have christened it the "October 2020 Update."
Microsoft is making today's test build available to Insiders in the Release Preview Channel if they manually seek it out and use Windows Update. To do this, Insiders need to go to Settings > Update & Security > Windows Update and then choose to download and install 20H2. Once they do this, they'll get new updates to this build through Windows Update. Microsoft made Windows 10 20H2 available for commercial customers who wanted to do pre-release validation starting in late August.
Officials are not yet saying when Microsoft will make Windows 10 20 H2 available to mainstream users who are not in the Insider program -- either by them seeking the feature update or via Windows Update.
Microsoft also is starting to roll out Windows 10 20H2 automatically to Insider testers in the Beta Chanel. Starting today, those Beta Channel Insiders who have not yet installed the 20H2 Update previously will be offered it automatically via Windows Update.
As Microsoft announced earlier this year, Windows 10 20H2 will look and feel like a smallish cumulative update for those already running the Windows 10 2004/May 2020 Update. (Microsoft describes this as delivery via an "enablement package," which is the same way the company updated users on Windows 10 1903 to 1909.) To those not yet on the 2004 update, the Windows 10 20H2 Update will look and act like a regular Windows 10 feature update when it's installed.
Officials said today there's one known issue in today's test build: Those in the Beta and Release Preview rings running 19042.508 will be impacted by "Element not Found" when they try to start Windows Subsystem for Linux. Microsoft officials said they have a fix which should be in the next 20H2 servicing release.
Microsoft officials have not said anything publicly about what happens after Windows 10 20H2, but according to the last I heard, Microsoft may be moving to a new timetable, via which the team only delivers one Windows 10 feature update per year (in the second half of the calendar year), and one Windows 10X feature release per year.