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This will switch from Microsoft Update back to using Windows Update. If it works for you fine; you can now reset the update settings to as they were earlier.
2] Run the Windows Update Troubleshooter from Microsoft.
Check this Fix It to reset Windows Update components and this Fix It to repair Windows Update components.
Please check which Fix It applies to your version of Windows. To manually download the latest definition updates for Windows Defender: When the definition file runs, a file extraction dialog box appears.
After the file extraction dialog box closes, verify that the Windows Defender definitions have been updated.
4] Use a standalone anti-malware to scan your system for malware.
Sometimes malware can turn off updating of security software.
5] Anand Khanse is the Admin of The Windows and a 10-year Microsoft MVP Awardee in Windows for the period 2006-16.
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.
Windows Defender in Windows 10 or Windows 8.1 is set to automatically download and install the definition updates using Windows Update, once a day.
If for some reason, your Windows Defender will not update automatically, or if you wish to download and save the definition updates in order to maybe update Windows Defender offline, on different installations of Windows 8, then this post will be of interest to you.