Microsoft is rolling out a new cumulative update for Insiders on the Release Preview ring on both PC and Mobile, featuring more under the hood improvements and fixes as most cumulative updates do. Microsoft is yet to publish an official changelog for this update, and considering this cumulative release is for Insiders only, it'll take a little longer to get a changelog up.
The update in question is referred to as KB3216755 on PC, and simply build 14393.726 on Mobile. It's great to see Microsoft continue to support Windows 10 Mobile and Windows 10 on PC with continued servicing updates and cumulative updates.
Stay tuned at Windows Central for more details regarding this new update. We'll be updating this article with a changelog once it becomes available!
Update: (1/27) Microsoft has now posted the changelog of what's improved and fixed in this release.