Update KB5050009 not installing.

So the manual installer says that update is installed. But Windows Update requires restart. I've done it several times but nothing has changed. It still requires restart...

I faced the same problem however I have had this problem back in 23H2 version.
The fix is pretty simple, you go to the settings, updates, in the bottom tabs there is "Pause updates for 1 week" Once you press it, you have to click on Resume updates and it scans the current version by verifying that you already have the KB5050009 Update installed and the issue is fixed.

If your PC was running Windows 10 previously and you did update it without performing clean installation with removing everything (including partitions)
Most probably your recovery partition is too small to run updates above 500 Megabytes (Since back in the days the updates were not larger than 500 Megabytes and as of today, they can reach up to 600 Megabytes). If that's the case you should extend the recovery partition size up to 642 Megabytes.

Seriously? Run unknown bat file, is this safely?

The batch file's contents are published on that link, a reputed site run by a very longtime MS-MVP.

3 years have passed since publication

That doesn't matter. The procedure for resetting WU has always been the same all these years.

Here are the official steps if you want to reset it manually. Ignore the regsvr32.exe part; they don't work in Windows 10 and 11.

https://learn.microsoft.com/en-us/troubleshoot/windows-client/installing-updates-features-roles/additional-resources-for-windows-update

Computer used: Dell XPS 13 9315, Windows 11 Professional 24H2 build 26100.2894

26100.2894 means the update has already been applied to your system.

January 14, 2025—KB5050009 (OS Build 26100.2894) - Microsoft Support: https://support.microsoft.com/en-us/topic/january-14-2025-kb5050009-os-build-26100-2894-bdbfb097-ea20-487d-9171-718d15e26f1b

If it's offered again, please try resetting the WU cache. https://www.elevenforum.com/t/reset-windows-update-in-windows-11.3808/

Seriously? Run unknown bat file, is this safely? 3 years have passed since publication

Computer used: Dell XPS 13 9315, Windows 11 Professional 24H2 build 26100.2894

26100.2894 means the update has already been applied to your system.

January 14, 2025—KB5050009 (OS Build 26100.2894) - Microsoft Support: https://support.microsoft.com/en-us/topic/january-14-2025-kb5050009-os-build-26100-2894-bdbfb097-ea20-487d-9171-718d15e26f1b

If it's offered again, please try resetting the WU cache. https://www.elevenforum.com/t/reset-windows-update-in-windows-11.3808/

Um.... I´m getting the error 0x8007139f

I´ve also tried to install the Updates Ramesh recommended.

But the first was not installable on my computer and the second mentioned to be already installed.

I´m using win11 x64 Build 26100.2894