Summary: Use Windows PowerShell 4.0 in Windows 8.1 to configure Windows Defender signature updates.

Hey, Scripting Guy! Question How can I use Windows PowerShell to configure Windows Defender signature updates?

Hey, Scripting Guy! Answer Use the Set-MpPreference function and specify a value for the SignatureScheduleDay parameter
          (to update signatures daily, use the value of 0):

Set-MpPreference -SignatureScheduleDay 0