I have been running Windows 10 build 9879 (the first public preview version) for quite some time on a secondary laptop. It ran pretty well.

Lately, Microsoft made available build 9926. I went to my upgrade settings to get the new build. It installed correctly (takes very long time). It restarted automatically to complete the upgrade and I was able to see the new build version at the bottom left of my screen. I was an happy guy.

All of a sudden, without touching anything, there was a message on the screen that the laptop needed to reboot to finish the upgrade. After the restart, I was back to build 9879. A roll back occurred without even asking me!

A few days later, retried the upgrade. Same scenario happened. The upgrade ran correctly (takes very long time), restarted and boom, ... rolled back.

I finally found a way of having a stable latest build: I have downloaded the ISO from my MSDN account and everything now runs smoothly.

So don't loose your time and your patience trying the upgrade. Download the ISO and you will be another happy previewer!