Your antivirus software - Windows Defender - sends automatic definition updates as part of your Windows updates. Windows sends updates automatically in most cases.  If you get notified that you have updates, you should always choose to run any essential Windows updates if you are prompted.  You can always check to make sure that your updates are up-to-date

Checking on Windows Updates

1.  Click on your Windows 10 icon and then click on the Settings icon.

2.  In the Windows Settings window, click on Update & Security.

3. In the Windows Update window, you can click on the Check for updates button..

Checking on Windows Defender updates

If your Windows updates are all set, but you still want to check to make sure that you have the latest antivirus definitions, you can check for updates at any time.

  1. Open Windows Defender in your applications list, or go to Settings>Update & security>Windows Defender>Open Windows Defender Security Center.  Click on Virus & threat protection.

2.  Click on Protection updates

3.  Click on the Check for updates button.

4.  When the system has finished checking for updates, you can close the window if no updates were found.