Microsoft today released Windows 10 Insider Preview build 17758 to the Slow ring, a build that's only been available in the Fast ring for three days. In fact, it's also only been three days since we last got a Slow ring build, a far cry from its normal cadence of monthly builds.
The reason for this rapid pace is, of course, that it's crunch time for Redstone 5, which is the codename for the next feature update to Windows 10, also known as version 1809 and the October 2018 Update. The general public will see this update next month, but Insiders are going to see an RTM build this month, possibly even next week.
There have been two builds since the last Slow ring build, and since Microsoft doesn't do Slow ring changelogs, here are the release notes for both of them:
For those that want to do a clean installation of the build, the team will probably release new ISO images next week. The current images are for build 17744.
You can install today's update through Windows Update, and if you're not on the Slow ring yet, you can go to the Windows Insider Program tab to get started.