The Windows Update is only a relatively small incremental update, but it will come with a number of new improvements and several new features.
Microsoft will begin updating devices with the latest hardware first, and then when version 1903 is stable, deployment will continue to other computers. Microsoft is expected to start rolling out the update on May 21, 2019, but it will be a gradual rollout and it could take weeks for all devices to receive the update. However, it's designed to perform an in-place upgrade only on devices where Windows Update doesn't work. If the tool doesn't work, try the Upgrade Wizard, which is a tool similar to the Media Creation Tool.