Windows Defender is supposed to automatically update itself. But I periodically experience these updates getting ‘stuck’. To force an update from the CLI:

cd “C:\ProgramData\Microsoft\Windows Defender\Platform”
dir
# REM cd into the latest iteration
cd 4.18.26060.3008-0
MpCmdRun.exe -removedefinitions -dynamicsignatures
MpCmdRun.exe -SignatureUpdate

• -removedefinitions -dynamicsignatures removes only the dynamic security intelligence/signature content already present on the box. In practice, this is a rollback/flush step. It is commonly used when you want to clear bad/poisoned/cached dynamic detections.
• -SignatureUpdate does the fetch/install step: it contacts the configured update source and downloads the current antimalware definitions/security intelligence.

If all else fails, download latest from https://www.microsoft.com/en-us/wdsi/defenderupdates and manually install it