When NVIDIA reinvented its SHIELD Tablet into the SHIELD Tablet K1 about a month ago and released it on the market, it promised that although the tablet was shipping with Lollipop, an update to Android 6.0 Marshmallow was coming before the end of the year. A month has passed and it seems that NVIDIA will make good on its promise as the update looks to be in its final testing stages.
The company posted a video sneak peak of Marshmallow on the Tablet K1, showing off the new app permissions, Now on tap, the launcher's vertical drawer and search bar, its own cloud gaming service GeForce Now, and focusing on the fact that Marshmallow will let you adopt MicroSD cards as internal storage for your tablet. The only NVIDIA-specific addition mentioned in the video is a new camera application that utilizes the K1 processor to add enhancements and effects to your photos.
The video's description on YouTube specifically notes that the update is coming to the Tablet K1 in December. That leaves about two weeks for the OTA to start rolling out to the device. The same update should come to the original SHIELD Tablet too, but we know that it'll take a little longer, so you shouldn't expect it before 2016.