Microsoft didn't put the link to the latest and required ADMX files. Please use https://www.microsoft.com/en-us/download/details.aspx?id=108428 as they contain the SecureBoot.admx/.adml required to set the GPO.

The changes have already been part of cumulative updates for a few months now. They are (as of now) just not getting applied to your firmware automatically in most cases. As of January and February CU, on select high-confidence devices they get automatically applied to firmware, and on machines where the updates have been applied on another way (e.g. by installing firmware updates or because the machine shipped with the new certificates) they have been made active in the February CU. Also, on client machines (Windows 10/11) that have telemetry enabled, you can also receive them via Controlled Feature Rollout if Microsoft deems them to be low risk for your machine.

Updating Secure Boot certificates consists of three mandatory and two optional steps:

• Add new certificates to the Secure boot Database (stored in your firmware's flash/NVRAM). This is currently done automatically on high-confidence machines as part of the cumulative update, and can manually be triggered by a registry switch.
• Add new Key Exchange Key (KEK) to the firmware's flash/NVRAM: This requires that the manufacturer of your firmware (the one holding the Platform Key) submits a signed update to Microsoft, which can then be applied to the flash/NVRAM like the step before. Windows updates include several hundred of such KEK updates, and if DB updated has been performed and such a KEK is available for your machine, KEK will be updated as well automatically. (There are various event log event IDs that are logged if this step fails due to missing signature or other reasons)
• Change the bootloader (bootmgfw.efi file) stored in your Efi System Partition (replace the one from c:\Windows\Boot\EFI by the one from c:\windows\Boot\EFI_EX). As of February CU, if the new certificates are in DB (on one way or other), the bootloader will be changed automatically.

Optional steps:

• Add the old DB certificates to the DBX blocklist. This is not applied automatically, only possible via registry key/GPO/... as it has the consequence that "older" Windows install/recovery media (i.e. including everything that you can download right now as .ISO from Microsoft) will stop booting on those machines without manual intervention
• Add extra SVN (Secure Version Number) update to DBX blacklist, blocking bootloaders older than mid of last year. Same as above, the impact would be too high to deploy them now automatically. If you are scared of Black Lotus, you can apply them

The reasons why this is done so cautiously are manifold:

• This is the first time those certificates are updated (since inception of Secure Boot in 2011). Therefore, some firmware implementations have bugs which may result them in freezing or requiring other manual attention when they try to install the Secure Boot updates. Therefore it is too high a risk for Micrsoft to install them unattended on all machines at once.
• Some machines even require a firmware update to make the process work flawlessly. This includes some virtualization solutions, e.g. Hyper-V will ship a fix in March 2026 CU which makes it possible to perform the process inside Hyper-V VMs that have Secure Boot and TPM both enabled. The fix needs to be applied to the Hyper-V host, not to the guests (so it is like a firmware update for the virtual firmware of the virtual machine).
• On some exotic machines where the manufacturers do not exist any longer or have lost their Platform Key secret part (yes also this happens), it will require manual intervention - either go to Firmware Setup and load the new KEK manually - if the firmware provides an option to do so - or install a firmware update. If neither option works for your device, and you have a sufficiently large amount of such devices (or sufficient free time) to justify the effort, it is also possible to set up your own root of trust (Platform Key) and install it to your firmware (unlike installing a KEK this option is required by Secure Boot certification. Of course it does not guarantee that it actually works). Probably something for tinkerers and not something you want to do in a professional/business environment.

Looking forwared to the event, although I will have to watch the recording as I don't have time to attend it.