Windows 11 September Update: What Changed and How to Check Installation

Microsoft released KB5124008 for Windows 11 24H2 and 25H2 on September 8, a cumulative security update. Here is how to identify your Windows version, install the update and check the result.

What has been fixed

Fixes include Teams and new Outlook closing unexpectedly on Arm64, black backgrounds caused by personalization problems, and incorrect cursor styles and colors.

Even if your computer seemed fine, the practical task is straightforward: check its update status and finish installation. An absence of visible problems does not tell you whether the relevant update is already installed.

Check your Windows version first

Open Settings → System → About. Under Windows specifications, find the version and edition: for example, 24H2 Home or 25H2 Pro. Your computer’s device name is not its operating system version.

The builds for this release are:

• 25H2: 26200.9445.
• 24H2: 26100.9445.

Note your version before starting so you can compare the result after restarting. “Windows 11” alone does not identify the appropriate update package.

Install the update and verify completion

1. Prepare your computer. Save open documents, plug your laptop into power and check your internet connection. Download time depends on your connection and the package size; leave more than a minute before your next call.
2. Check for updates. Microsoft’s instructions take you to Settings → Windows Update → Check for updates. Select the download and installation option if updates are available.
3. Complete installation. If Windows requests a restart, do so after saving your work. A pending restart means installation is not finished.
4. Verify the result. Return to Windows Update, open Update history and look for a successful KB5124008 installation. Then compare the build under About with the appropriate number above.

Use the built-in Windows Update service for this check. Avoid update packages linked from unsolicited messages or advertisements. If you read this later, consider the current Windows Update status too, rather than relying solely on a September build number.

If installation fails

Note the error code and open the Windows Update troubleshooter in Get Help. Back up important files before attempting further repairs. For a work PC, give your administrator the Windows version, error code and time of the failed attempt; that is more useful than simply reporting that updates do not work.

A separate deadline for 24H2 Home and Pro

Microsoft’s schedule ends updates for these editions on October 13, 2026. Installing September’s package does not replace moving to a supported version. Check Windows Update for an offer to upgrade to 25H2; coordinate the move with IT on a managed computer.

Check your browser separately too: our article on completing a Chrome update walks through that part of the process.