It's code-named Threshold Wave 2, and will be the first major update to the operating system, larger than the smaller patches Microsoft regularly releases. It's not at all clear what will be in Threshold 2, but some believe that it may bring promised extension support to the Edge browser.
Microsoft will make Threshold 2 beta builds available earlier than its expected final October release date, although no date has yet been set. But if you want to be a beta tester, it's easy to do -- just make a few simple changes in Windows 10. Be aware, though, that the beta, particularly the earliest versions, could hose your machine. If you're willing to take the gamble, here's how to do it.
First, don't do this on your main machine, because of the dangers of using beta. Regularly back up any data on the machine you'll use for the beta, because you could lose it all.
Next, make sure that your machine is up to date with the latest Windows 10 builds. To do it, go to Settings-->Update & Security-->Windows Update-->Check for Updates, and download and install any updates. Make sure to continue to keep your machine up to date in this way.
Now go to Settings-->Update & Security-->Windows Update-->Advanced options. Scroll down to the "Get Insider builds' section and click "Get started." You'll be warned that "You will be installing pre-release software and services that might not be fully tested." Click Next if you're willing to be a guinea pig. After that get another, more dire warning that "If you ever want to stop receiving Insider builds you may need to remove everything from your PC and reinstall Windows." If that still doesn't deter you, click Next, then reboot.
At that point, whenever beta of Threshold Wave 2 is ready, you'll be in the fast lane for getting it.