We noted last week that Forza Horizon 3 had become available for preloading, but that preloading wasn't working for everyone. Preloading is now over—the game came out a couple of days ago, and by the way it's good—and so the update released yesterday may not seem like the timeliest fix ever. The issue apparently impacts a number of other games on the Microsoft store as well, though, including the now-preloadable Gears of War 4, so you should probably grab it anyway.
The patch notes on Microsoft's website don't refer to any specific titles, saying only (and among other things) that it will provide "improved reliability of downloading and updating games from the Store." But Xbox mainguy Larry Hryb (Major Nelson to his friends) said on Twitter that affected games include Halo 5: Forge, ReCore, and Gears of War 4, along with Forza Horizon 3. Gears of War studio The Coalition also urged Windows 10 owners to install the update before beginning Gears 4 preloads, which went live last night.
PSA for Win 10 users playing H5 Forge,ReCore,FH3+GoW 4 Pre-Order: Take update KB3194496 to address d/l issues https://t.co/SxW13FTKvT (1/2)September 29, 2016
The update should install automatically, if you have automatic Windows updates enabled; if not, go to Settings, then Update and Security, and then the Windows Update tab, at which point you'll want to click the "download" button. Be aware that if you install a language pack after this update, you'll need to reinstall the update. I have no idea why.