Today is a busy day for Windows Insiders, as Tuesdays have been lately. Microsoft released Windows 10 build 17763 to the Fast ring, and ISO images for build 17758 are available. However, the Slow ring is getting some love as well in the form of a cumulative update. KB4462120 is available, and that will bring the build number to 17758.1004.
The new cumulative update doesn't seem to include many actual fixes. The ones that it does include are those that are found in Windows 10 Insider Preview build 17760. But the reason Microsoft didn't just promote build 17760 to the Slow ring is because it needs to test servicing on the October 2018 Update.
This is something that happens before every feature update is released. Cumulative updates really need to be tested to a mass audience before a feature update rolls out. If something were to happen where cumulative updates failed, it would be a disaster.
If you're on the Slow ring, you can grab today's update through Windows Update. If not, you can go to the Windows Insider Program tab to get started.