According to Microsoft, it is taking a "measured and throttled approach" with the May 2019 Update, which means that not everyone will get the update at the same time. However, once Microsoft is able to study download/installation telemetry data and has a high confidence level that things are going smoothly, it will make the software available to all Windows 10 users.
If you want to manually force the installation of the May 2019 Update, there's a rather simple way to get it. Simply navigate to Settings --> Update & Security --> Windows Update and then select Check for updates. After the system queries Microsoft’s servers, you will likely see the option to Download and install the May 2019 Update. As we previously mentioned, Microsoft is taking a measured approach with the availability of this major update, so don't be alarmed if you don't see the option right away.
We can't blame Microsoft for being more cautious with the May 2019 Update. The company was burned quite badly by the Windows 10 October 2018 Update, which was riddled with bugs including data-killing goof-ups that left a bad taste in the mouths of consumers. So, if you're not willing to give Microsoft the benefit of the doubt and are more apt to wait before installing the May 2019 Update, we wouldn't fault you for erring on the side of caution.
For those that aren't already familiar with the Windows 10 May 2019 Update, it brings a number of new features to the table including more granular controlof Windows Updates, a new Windows Sandbox feature to isolate potentially damaged software downloads, the decoupling of Cortana from Windows Search, improved native fixes for Spectre and (via Retpoline), a new light desktop theme, a refreshed Start menu, and enhanced support for password-less logins.
Another big change is that Windows 10 will now reserve 7GB of disk space for future Windows Updates. This is likely the main reason why the system requirements for the May 2019 Update have increased for the first time since Windows 10 was released in 2015. Microsoft now requires at least 32GB of free storage space for both 32- and 64-bit versions of the operating system.