Update Defender definitions automatically

Q. How can I manually update definition files for Windows Defender?

A. Normally the definition files for Windows Defender will be updated as part of Microsoft Update, WSUS or another tool. You can also force definition update using PowerShell using (you can force a different source, for example -UpdateSource MicrosoftUpdateServer to use Microsoft Update or you can specify another location such as a file share using FileShares or Microsoft Malware Protection Center using MMPC):

Update-MpSignature

It is also possible to download the definition file directly and then execute the downloaded mpam-fe.exe file: