In the Win X Menu, click on Command Prompt (Admin) to open an elevated Command Prompt.
Type cd /d “\Program Files\Windows Defender” into the Command Prompt and press the Enter key.
Please read the entire post & the comments first, create a System Restore Point before making any changes to your system & be careful about any 3rd-party offers while installing freeware.
Many Windows 10 users have complained of Windows Defender – the Operating System’s resident antivirus programs – not updating regardless of whether the update being downloaded and installed is a small Definitions update or a large, chunky update. This works in most cases, where the issue is originated due to a system corruption.
Error Codet install - Microsoft Community In the past I had Defender not update a couple times.