The manufacturer is currently beta testing Android 6.0 for the Samsung Galaxy S6 and Samsung Galaxy S6 edge. These 2015 models will be updated to Marshmallow along with the Samsung Galaxy S6 edge+ and the Samsung Galaxy Note 5. All four models should also be in line to receive Android N along with the Samsung Galaxy S6 Active.
We don't know exactly when Sammy will start rolling out Android 6.0. Considering that the Samsung Galaxy S7 will probably be unveiled during the February 21st Unpacked event (the day before MWC 2016 opens in Barcelona), we could see the update start to appear sometime next quarter.
You can check out a series of photos showing the Samsung Galaxy S6 and Samsung Galaxy S6 edge running Android 6.0, by clicking on the slideshow below.
source: SamMobile (1), (2)