January 2018 update for Windows 10 Mobile is now rolling out
Build 15254.158 released for the latest version of Windows 10 Mobile
Microsoft released the January 2018 update for Windows phones on the Fall Creators Update (branch 1709). It comes with build 15254.158, which, according to the Microsoft Support page, includes quality improvements, like security updates to Microsoft Edge, Windows Kernel, and other elements of Windows. Meltdown and Specter security flaws found in all modern processors should also have been fixed in this update.
Windows Phones on the Creators Update (version 1703) will also receive updates to build 15063.850, while those on the Anniversary Update (version 1607) will get build 14393.2007.
Like any other cumulative update, these builds bring nothing more than improvements and fixes.
You can download and install the security and performance updates from Settings → Update & Security → Windows Update.