Other than the built in feedback app - because it has a bug for me on build 10166 and is just a blue windows with a logo that doesn't provide any means to do anything with it.
On build 10166, I'm finding there to be many bugs. Some of them are:
- When my PC goes into hibernation, it automatically re-awakens at random intervals, from 30 seconds up to 30 minutes later
- The horizontal / vertical / diagonal windows resize cursors that appear when targeting the corners of windows are often misaligned in many cases, meaning I have to target background space, or a bit higher, or a bit lower, to grasp the window as desired and resize it
- sometimes after changing a setting, the start menu won't reappear when clicked on for a long while
- a certain custom theme isn't loading for me, despite working for others, and attempting to load it crashes any instances of File Explorer which I have open. If others want to try loading that theme, it's here: Vista Themes for Windows 10 by sagorpirbd on DeviantArt
- changing program defaults for web browser doesn't work, and everything keeps launching in Edge
- changing program defaults for media doesn't work
I presume the last two issues could be by design of MS to test Edge and their media player (which is visually dull, and slow), but there's nothing directly indicating that.