https://arstechnica.com/gadgets/2019/05/windows-10-may-2019-update-now-rolling-out-to-everyone-slowly/

To avoid a replay of the problems faced by the Windows 10 October 2018 Update, version 1809, Microsoft has taken a very measured approach to the release of the May 2019 Update, version 1903, with both a long spell as release candidate and a much less aggressive rollout to Windows Update.

That rollout starts today: while previously one needed to be in the Insider Program (or have a source such as an MSDN subscription) to download and install version 1903, it's now open to everyone through Windows Update.