Late September saw the release of the latest Windows 10 Insider Preview Build (14931) for PC to Windows Insiders in the Fast ring.
If you are on an Insider program, it is important to remember about build expiration. Since 15 September, PCs running older Insider Preview builds may start to see build expiration notifications daily. Even better news (!), from the start of October, these PCs will start rebooting every three hours and then on October 15th these PCs will stop booting all together. This doesn’t sound like a good thing…
If your PC is running one of these old Insider Preview builds, please make sure you update to Build 14926 or newer via Settings > Update & Security > Windows Update. Builds 14926 and newer have an updated expiration date of 1 May 2017. If your PC is running the Windows 10 Anniversary Update (Build 14393) in the Slow and Release Preview rings today, this does not apply and you will not be notified of your build expiring.
You can read more here.