The cumulative update not only knocks out PCs' default settings, it prevents users from resetting them.
If you have Chrome as the default browser on your Windows 10 computer, you'd better check to make sure Microsoft didn't hijack it last week and set Edge as your new default. The same goes for any PDF viewer: A forced cumulative update also reset PDF viewing to Edge on many PCs.
Do you use IrfanView, Acdsee, Photoshop Express, or Elements? The default photo app may have been reset to -- you guessed it -- the Windows Photos app. Music? Video? Microsoft may have swooped down and changed you over to Microsoft Party apps, all in the course of last week's forced cumulative update KB 3135173 .