According to Sprint, it "can take several days" before all owners of the G Stylo are getting the Marshmallow update, so you should be patient if you're not receiving it today.
Released back in June by Sprint, the LG G Stylo currently costs $288 off contract, or $49 with a 2-year contract agreement. The handset is a pretty large one, as it sports a 5.7-inch display with 720 x 1280 pixels. Other features include a stylus pen, LTE connectivity, 5 MP front-facing camera, 8 MP rear camera, 1 GB of RAM, and 8 GB of expandable internal memory. The G Stylo is powered by a quad-core Qualcomm Snapdragon 410 processor, and includes a 3000 mAh battery. To find out more about the handset, check out our LG G Stylo review.
We assume that T-Mobile's LG G Stylo (which has been available since May) will also be updated to Android Marshmallow, though there's no word on when that could happen.
source: Sprint via Android Police