There's no changelog at the time of writing, so assume this is just a collection of bug fixes and stability improvements. Which are much needed, of course, even on the much more stable Threshold branch. We still have a good month or two before Microsoft starts to think about pushing the 'Redstone' (14xxx) branch to production devices, so why not crack down on the smaller and more annoying bugs that remain?
You can update, as usual, by going into Settings/Update & Security/Phone update and 'Check for updates'. Make sure your battery is above 40% and/or plug the phone in!
In addition to older Windows Phones upgraded to Windows 10 Mobile, by necessity, through the Insiders programme, this update may also be of interest to all those who upgraded officially or who have a Windows 10-native phone (e.g. Lumia 950) and who want the latest bug fixes but who don't feel ready for the full 'alpha' nature of the Redstone builds!
I'm updating a test Lumia 920 and 1020 (I know, I know) as I speak, your comments welcome if you have anything to add for this specific build.