Microsoft begins the automatic rollout of the Windows 10 May 2019 Update, version 1903, on devices running Windows 10 version 1803, April 2018 Update using the Windows Update system.
Although starting with the May 2019 Update, new versions of Windows 10 no longer download and install automatically, devices nearing the end of service will continue to receive feature updates automatically to ensure they stay supported and continue to receive monthly updates.
The April 2018 Update will reach the end of 18 months of service on November 12, 2019, and as a result, starting July 16, 2019, Microsoft will begin the update process for Windows 10 Pro and Home editions using the same auto-update mechanism prior to the release of the May 2019 Update.
If you’re still not ready to upgrade, you can pause update for up to 35 days from the Windows Update settings page. In the case that you need more time, you can use this guide to block the automatic update until you’re ready.