This is pretty stupid to tie in updates for the OS to the updates for the security software (when the former happens once a month and the latter one -- once a day.) But, we're dealing with Microsoft ...
I couldn't seem to find an easy way to make Windows Defender auto-update itself on my Windows 8.1, so I came up with this solution:
Go to Control Panel -> (Switch to classic view) -> Administrative Tools -> Task Scheduler.
Set up a recurring daily task to run the following command (with double-quotes):
"C:\Program Files\Windows Defender\MpCmdRun.exe"
with the following arguments:
I personally set it to run at 9 AM daily, but you can pick your own time.
PS. I know, Microsoft Task Scheduler is an
obtuse creature that needs a long tutorial of its own, but the good thing is that you have to deal with it only once.