As a Windows Insider you can already use the April 2018 Update. The insider program is for testers and Windows fans who always want to have the latest features. Some pre-release versions of Windows 10 have annoying bugs, so an insider version should not actually be used productively. Currently, however, Microsoft is providing the upcoming release via the Insider program.
So, sign up for the Insider program and set your Microsoft account under "Update and security" under "Windows Insider Program" on. Select "The next version of Windows" and restart the computer. After that, you'll be served the latest build of the Spring Creators Update.