Mail Alert is easy to install and just as easy to configure as shown below in the Mail Alert configuration window.
Once the update package has been downloaded, open it to begin updating Windows Defender to its latest iteration.
If the issue is with your computer or a laptop/notebook you should try using the Reimage Plus Software which can scan the repositories and replace corrupt and missing files.
Right-click on the Start Menu button to open the Win X Menu.
In the Win X Menu, click on Command Prompt (Admin) to open an elevated Command Prompt.
This will initiate a Windows Defender update which, hopefully, will be successful.