NVIDIA sent out the first Marshmallow update to the SHIELD Tablet in late 2015 with the SHIELD K1 OTA. It said at the time that the original SHIELD would be updated in early 2016. That was kind of true—a few months later the update started going out to the WiFi-only SHIELD. However, the US LTE SHIELD is only now getting the v4.1 update. Better late than never, I suppose.
The OTA started rolling out yesterday and should be available immediately on the LTE SHIELD in the US. Here's what it includes.
Supports the new Vulkan™ API for high-efficiency access to SHIELD’s graphics hardware
Adds Android 6.0.1 changes
Repositioned Navigation Buttons in Landscape mode & Updated Emojis
Double Tap Power Button to launch SHIELD Camera app
Do Not Disturb Until Next Alarm
Support for Android Doze mode & additional power optimizations
Adds USB MIDI support
Adds memory compression enhancement
Improved Wi-Fi connectivity when waking from sleep
Fixed Compass functionality
Fixed Gamepad Mapper issues
Fixed issues with moving apps to SD card
Fixed Doom 3 HDMI output issue
Repositioned Lasso Capture icons in landscape mode
Update to Android 6.0 Security Patch Level March 1, 2016
Overall stability and security enhancements
The update is about 735MB, and takes the tablet all the way from 5.1 to 6.0.1. The delay was due to carrier certification, something you don't usually have to deal with on unlocked devices. For whatever reason, NVIDIA gave AT&T veto power over updates on the US LTE version of the tablet. At least now the OTA is out.