If you are having trouble trying to update your windows 10 definition here i show a simple way o fix this windows defende update error : 0x80070643 KB226760
FIX: I was able to fix this by opening CMD as Admin(right click open as administrator) and typing (with quotes):
"%PROGRAMFILES%\Windows Defender\MPCMDRUN.exe" -RemoveDefinitions -All
All Then I typed (with quotes:
"%PROGRAMFILES%\Windows Defender\MPCMDRUN.exe" -SignatureUpdate
Hope this helped!