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What's new in KB5120998 (Windows 11 build 26100.9278)

Affects
  • Windows 11, version 25H2
  • Windows 11, version 24H2
  • OS Build 26200.9278
  • OS Build 26100.9278
Priya Natarajan4 min read

Quick answer

KB5120998 is an optional preview update released on August 27, 2026, for Windows 11 version 24H2 and version 25H2. It updates your system to OS Build 26100.9278 or OS Build 26200.9278, bringing non-security fixes, quality refinements, and system reliability improvements ahead of the next scheduled monthly update. You can install it manually through Windows Update under optional updates.

On this page

Microsoft released KB5120998 on August 27, 2026, as an optional non-security preview update for PCs running Windows 11, version 24H2 and Windows 11, version 25H2. Installing this update bumps your system operating build to OS Build 26100.9278 on version 24H2 setups and OS Build 26200.9278 on version 25H2 setups. Because this update is part of the monthly optional preview cadence, it is designed for users who want to test early fixes and maintenance patches before they are bundled into the mandatory Patch Tuesday update in the following month.

Preview updates like KB5120998 do not install automatically unless you have configured your Windows Update settings to receive the latest updates as soon as they become available. This build focuses primarily on background platform stability, non-security quality fixes, and servicing improvements across supported hardware configurations. Below are the key changes and release details included in KB5120998:

  • Released on August 27, 2026, as an optional preview cumulative update.
  • Updates Windows 11, version 24H2 devices to OS Build 26100.9278.
  • Updates Windows 11, version 25H2 devices to OS Build 26200.9278.
  • Delivers underlying OS servicing maintenance and performance stability fixes.
  • Previews non-security quality improvements slated for the upcoming general release.

If you want to install KB5120998 or verify whether your PC is already running OS Build 26100.9278, follow the step-by-step methods below.

Method 1: Install KB5120998 via Windows Update

The simplest way to get KB5120998 is directly through the Windows Update interface in your Settings menu. Since preview updates are optional, you must manually initiate the download.

  1. Go to Settings > Windows Update by opening your Start menu and selecting the gear icon, or by pressing Windows key + I.
  2. Turn on the toggle switch next to Get the latest updates as soon as they're available.
  3. Click the Check for updates button in the top-right corner of the window.
  4. Wait a few seconds for Windows to query the update servers.
  5. Look for KB5120998 listed under the Optional updates available section.
  6. Click Download and install next to the update listing.
  7. Allow the installation progress bar to reach 100 percent.
  8. Click Restart now when the system prompts you to finish applying changes.

Use this method when you want to get early access to the latest fixes before mandatory updates arrive. Afterwards, your system will restart and run on OS Build 26100.9278 or OS Build 26200.9278.

Method 2: Check your Windows 11 build number

If you are unsure whether KB5120998 is already installed on your computer, you can verify your build number in System Settings or through the Windows Version diagnostic tool.

  1. Go to Settings > System > About by navigating through the main Settings app menu.
  2. Scroll down the right pane until you see the Windows specifications section.
  3. Look at the OS build entry listed under specifications.
  4. Confirm whether the value displays OS Build 26100.9278 or OS Build 26200.9278.
  5. Press Windows key + R to open the Run dialog box as an alternative check.
  6. Type winver into the Open field.
  7. Click OK or press Enter on your keyboard.
  8. Read the pop-up dialog box to inspect the version and OS build text shown on screen.

Use this method to confirm whether your system has applied the August 27, 2026 preview update. Afterwards, you will know exactly which update level your device maintains.

Method 3: Run the Windows Update Troubleshooter

If KB5120998 fails to download, gets stuck during installation, or throws an error code, running the built-in troubleshooter can resolve underlying service glitches.

  1. Go to Settings > System > Troubleshoot by opening the main Settings menu.
  2. Click Other trouble-shooters from the list of system diagnostics options.
  3. Locate the Windows Update option near the top of the troubleshooter list.
  4. Click the Run button next to Windows Update.
  5. Wait while the automated system checks for damaged update files and reset parameters.
  6. Apply any recommended fixes offered by the diagnostic wizard.
  7. Close the troubleshooter window once the diagnostic process finishes.
  8. Go back to Settings > Windows Update and click Check for updates again.

Use this method whenever an optional update stalls or refuses to complete. Afterwards, core update components will reset, allowing KB5120998 to download cleanly.

If nothing worked

If KB5120998 still fails to install after running the troubleshooter, keep in mind that preview updates are strictly optional. You can safely skip KB5120998 and wait for the official, mandatory Patch Tuesday cumulative update, which will automatically bundle all fixes included in this release.

If you prefer to resolve the issue immediately, try pausing updates temporarily. Go to Settings > Windows Update, click Pause updates for 1 week, restart your computer, and then click Resume updates to refresh your update catalog connection. Alternatively, running the System File Checker tool via Command Prompt (run sfc /scannow as Administrator) can fix corrupted system files that prevent new OS builds from applying successfully.

Frequently asked questions

Is KB5120998 a required update for Windows 11?

No, KB5120998 is an optional preview update released on August 27, 2026. You do not have to install it unless you want early access to bug fixes before the next mandatory update release.

What build version does KB5120998 bring to Windows 11?

KB5120998 updates Windows 11 version 24H2 systems to OS Build 26100.9278 and version 25H2 systems to OS Build 26200.9278.

How do I check if KB5120998 installed successfully?

Go to Settings > System > About or press Windows key + R, type winver, and check if your build number displays 26100.9278 or 26200.9278.

Verified: pending — this guide is awaiting a hands-on check on a current build.

Written by

Priya Natarajan

Writes Thea's how-to guides for new and changed Windows 11 features: where a setting moved, how to turn it on or off, and what it actually does. Every guide is checked against Microsoft's release notes and updated when a new build changes the steps.

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