Microsoft has begun pushing out Windows 10 Mobile build 14393.351 to the Release Preview and Slow Rings. So far, the update is only showing for Mobile and not on PC.
No changelog is yet available or announced on the Feedback Hub although we'll keep an eye out for anything official. As usual, this update just contains various bug fixes and no new features.
This update replaced the current 14393.321 that is available for Release Preview and Production.
Note: For those on the Fast Ring Microsoft is aware of the issue of the update being stuck at 0%. There is no current workaround, so sit tight until they figure it out. That bug may apply to this update as well, so do not be surprised if you are stuck at 0%.