How to fix install error 0x800f0991 in Windows 11, version 25H2

Please try performing an In-place upgrade. It will definitely help in resolving update errors. Performing this can repair broken operating system files while your personal files, settings and installed applications will not be removed.Few things to check on the PC

• Unplug all external peripherals except for the Mouse, Keyboard, and LAN cable before starting
• Disable or uninstall any third-party Antivirus program to prevent it from interfering with the upgrade
• Make sure you have enough space on your Windows drive.

On your working PC, open the official Microsoft Windows 11 download page.

https://www.microsoft.com/software-download/windows11

Choose Download Windows 11 Disk Image (ISO).

Select the edition and language, then click Download and save the .iso file to your PC.

Right-click the downloaded .iso > Mount (or double-click). A new drive letter appears.

Open that virtual drive and run setup.exe.

When prompted, choose Keep personal files and apps (or choose a different option).

Follow on-screen prompts: accept license, select options, the installer will copy files and restart several times.

I look forward to receiving your update and I'm willing to collaborate with you till we resolve the issue. Do keep me informed by clicking "Add comment" below.