"Starting today, we are giving Windows Insiders in the Dev Channel the option to receive builds from the RS_PRERELEASE branch. These builds will be offered as an optional update through the “seeker” experience in Windows Update. This means Insiders will have to go and choose to install the currently offered RS_PRERELEASE build.
Insiders can choose to install Build 21277. Once they do, they will continue to receive RS_PRERELEASE builds going forward.
Insiders who choose to stay put will continue to automatically get builds from the FE_RELEASE until we move all Insiders in the Dev Channel to receiving RS_PRERELEASE builds sometime after the holidays. We still need Insiders running FE_RELEASE builds and giving us feedback on these builds so its ok if you choose to stay on these builds.
Be sure to check the list of known issues below. The list of fixes and known issues between FE_RELEASE builds and RS_PRERELEASE builds will NOT be the same."