Last week, Microsoft fully unveiled its suite of new icons, rolling them out to Windows Insiders on the Fast ring. The icons are based on the Fluent Design System, similar to the new Office icons that rolled out last year. But now, all of Microsoft's first-party apps are getting redone.
Today, users are spotting that some of those icons are now rolling out to non-Insiders. Specifically, it seems like the Mail & Calendar suite of apps is getting updated. Presumably, the other apps will follow soon, being that there aren't any other breaking changes being tested with Insiders.
One thing that there's been a bit of backlash on is that the tiles for the new icons are blue, rather than transparent. That means that the tile background won't change to whatever your Windows 10 accent color is. At least for Mail & Calendar, that has not been changed for the Production version of the apps.
If you want to update your app, you can get it now. Just open the Microsoft Store, go to Downloads and updates, and clock on Get updates.
Thanks to Shawn for the tip!