Update from Microsoft protects Windows 10 Mobile handsets from Meltdown and Spectre
Microsoft has pushed out a cumulative update for all Windows 10 Mobile handsets regardless of whether they are registered with the Insider program or not. On the company's support page, it says that the update (KB4073117) includes all of the improvements to the OS found in the KB4056891. That means it contains security updates to Internet Explorer, Microsoft Scripting Engine, Microsoft Edge, Windows Graphics, Windows Kernel, Windows Subsystem for Linux, and the Windows SMB Server.
While this update is being sent OTA to Windows 10 Mobile devices, you can see if it has hit your phone by going to Settings > Update and Security > Windows Update. Before installing the update, make sure that you are connected to a Wi-Fi signal, and that the battery on your phone is fully charged.
source: Microsoft via SurfacePhone.It (translated), MSPoweruser