If you're currently running Windows 10 Version 1507, Version 1511, Version 1607, or Version 1703, you can expect to receive a notification that states that your device has to have the latest security updates installed. Windows Update will then try to update your device.
After you receive the update notification, be sure to leave your PC plugged in and powered on while the download completes. Restart your PC when prompted.
This update is also offered directly to Windows Update Client for some devices that have not installed the most recent updates.
Windows 10 Version 1507, Version 1511 and Version 1607 are currently at "end of service." This means that devices that are running these operating systems no longer receive the monthly security and quality updates that contain protection from the latest security threats. To continue receiving security and quality updates, Microsoft recommends that you update the system to the latest Windows version, Windows 10 Version 1803.
Windows 10 version 1703 is not yet at "end of service." However, it must be updated to the latest versions of Windows 10 to ensure protection from the latest security threats.
Microsoft is aware that this notification was incorrectly delivered to some Windows 10 Version 1703 devices that had a user-defined feature update deferral period configured. Microsoft mitigated this issue on March 8, 2018.
Users who were affected by this issue and who upgraded to Windows 10 Version 1709 can revert to an earlier version within 10 days of the upgrade. To do this, open Settings > Update & Security > Recovery, and then select Get started under Go back to the previous version of windows 10.