Marshmallow has been out for about six months now, and there are still some very recent phones that don't have the update. For example, the T-Mobile Galaxy S6 and S6 Edge... until today. These devices are finally getting the Marshmallow OTA. Sprint and Verizon both rolled their GS6 updates out recently, leaving AT&T as the sole holdout.
The update clocks in at a hefty 1.3GB and makes some changes to the default app loadout. Facebook Messenger, WhatsApp, and Instagram are no longer built-in. There's also no decryption function in Marshmallow, so if you update while encrypted, that's the way your device will have to stay. Since it's 6.0.1, you also get the new emoji (yay taco).
You can head to the update menu to try and trigger the OTA. If it's not showing up yet, give it a day or two.
- Thomas Hunsaker and Robert Patrick