Trouble shoot Windows Update error 0x800F081F in Windows 8 / Windows RT
“If you receive Windows Update error 0x800B0100 or 800B0100, it means that a file needed by Windows Update is damaged or missing. Windows has troubleshooting tools that can help you address this problem. Follow the steps below to use these tools.
Run the DISM tool
Swipe in from the right edge of the screen, then tap Search.
(If you're using a mouse, point to the top-right corner of the screen, move the mouse pointer down, then click Search.)
Enter command prompt in the search box.
In the list of results, swipe down on or right-click Command Prompt, then tap or click Run as administrator.
In the Administrator: Command Prompt window, type the following commands. Press the Enter key after each command:
DISM.exe /Online /Cleanup-image /Scanhealth
DISM.exe /Online /Cleanup-image /Restorehealth
To close the Administrator: Command prompt window, type Exit, then press Enter.
Run Windows Update again.