Hi.
Windows Security is built into Windows and you cannot re-download it.
Windows Security doesn't install on your computer like a typical program, so you can't reinstall it like regular software. Follow the instructions below to use PowerShell commands to get the job done:
Run Powershell as administrator, enter:
Get-AppxPackage Microsoft.SecHealthUI -AllUsers | Reset-AppxPackage
Press Enter to immediately start the process of resetting Windows Security to its original settings so that it executes as a newly installed application.
Hope the information is helpful.
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