In case you still had doubts that build 15063 was "RTM", Microsoft has today pushed said build to the Release Preview ring on PC, meaning build 15063 will indeed be the build that rolls out to the public starting April 11.
The Release Preview ring is the final Insider ring before the production channel of updates, where Microsoft is expected to start rolling out build 15063 on April 11. Microsoft announced today that it would allow non-insiders to upgrade to the Creators Update early, starting April 5th via the Update Assistant tool.
Build 15063 is now being made available to the Release Preview ring for PC Insiders. Welcome to the Creators Update! https://blogs.windows.com/windowsexperience/2017/03/20/announcing-windows-10-insider-preview-build-15063-pc-mobile/Build 15063 is now being made available to the Release Preview ring for PC Insiders. Welcome to the Creators Update! https://blogs.windows.com/windowsexperience/2017/03/20/announcing-windows-10-insider-preview-build-15063-pc-mobile/— Windows Insider (@windowsinsider) 30 March 201730 March 2017
If you'd rather wait, you can expect Microsoft to begin the official rollout on April 11, and will rollout in waves over the course of 2017. Microsoft will target a whitelist of newer devices first, and slowly roll out to older devices over the next several months.
If you're an Insider in the Release Preview ring, you can expect to see build 15063 making its way to you now on PC. It'll install much like any other update, but may take a little longer than usual to download and install.
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