Here's how you update Microsoft Defender within a Windows 10 image

[Windows Central]

You can update Microsoft Defender inside a Windows 10 installation image, and in this guide, we'll show you the steps to complete this task.
Although after the installation of Windows 10, the built-in version of Microsoft Defender Antivirus will download and apply the most recent updates automatically, the process does not happen immediately, leaving the system temporarily vulnerable to attacks during this period.

Full story from the WindowsCentral blog...