If Windows Update can’t install a particular update (or isn’t working all), Microsoft’s DISM command provides a simple way to get Windows updates installed.

DISM Failed with Error: 0x800f081f The source files could not be found. Use the “Source” option to specify the location of the files that are required to restore the feature.

To fix Windows Defender from closing immediately after opening, you need to use the System File Checker, run the DISM tool, disable third-party antivirus, or troubleshoot in the Clean Boot state.