Windows Defender is Anti-Malware software built into Windows 10 and it protects users against all malicious software like Spyware, Adware, Virus, Worm, Trojan, Rootkit, Bootkit, Backdoor, Ransomware and so on. Normally, Windows Defender should work without problem in the background and just notify users when something is wrong or malware has been detected. However, in some cases, you might face problems with updating Windows Defender and with its update failing. There are three ways to update Windows Defender:

  1. Windows Defender Update Interface: When you open Windows Defender, you may open Update tab and then click on update and it downloads. Install all updates.
  2. Windows Update: Windows Update download and install updates for Windows automatically. When downloading updates, it will download and install updates for Windows Defender too.
  3. Manual Download from MMPC website: Users may download updates manually from Microsoft Malware Protection Center (MMPC) website.

If your PC is part of company's network or managed by an administrator, then they might configure updates through other means like local network drives or set policies to only download and install updates after approval. If you are part of company's network, discuss issues with updates with your network administrator's first. If you face any issue with updating Windows Defender, take a note of the update error and error code and follow these steps:

  1. Check and see whether you have any other Anti-Malware software installed or not? If you have other Anti-Malware software, it will turn off Windows Defender and disable Windows Defender update. To use Windows Defender, you will need to uninstall your current Anti-Malware software.
  2. Check for updating in Windows Defender's Update interface and if it failed, try Windows Update.
  3. If it didn't work, make sure to install all updates (if any) and restart your PC and try the update again.
  4. If problems persist, try Download and run the Windows Update Troubleshooter.
  5. If that didn't solve the problem, open Start and search for Feedback and open Feedback Hub app. Search for this issue and if it already exists, upvote it and if not, then add new Feedback. Then post your issue in Microsoft Answers forum