Microsoft has officially started to roll out Windows 10 version 1903, called the May 2019 Update, to everyone. Originally released to Insiders for testing on April 8th, Microsoft has now made it available to all users. Microsoft has stated that they are throttling its availability so it may not be listed when you check for updates. Starting in June, Microsoft will begin updating Windows version 1803 (April 2018 Update) and earlier to version 1903. Older Windows 10 versions to be auto-updated in June.
Source: https://www.bleepingcomputer.com/news/microsoft/microsoft-releases-windows-10-version-1903-may-2019-update/