Samsung have announced that Galaxy S6 and Galaxy S6 Edge users in UK and South Korea can beta test the Android 6.0 Marshmallow update if they’d like. This is the first time Samsung has taken such an initiative.
If you are Galaxy S6 or Galaxy S6 Edge user in UK or South Korea go to Settings >> About device and check the model number. You will have to be on an unlocked version of the device with model number SM-G920F or SM-G925F. If you are on one of the mentioned model numbers, follow the guide to get Android 6.0 Beta.