The updates for Windows Defender are delivered through Windows Update. I didn't see any update that can be downloaded manually, which I believe this happens due to security reasons.
Make sure that your Windows Update is working normally, then search manually for updates. If there are any, it will be downloaded for your software.
Use the troubleshooting page to diagnose and fix possible Windows Update errors: https://support.microsoft.com/en-us/gp/windows-update-issues/en-us