As we know Samsung has launched ‘Marshmallow Beta Program‘ and recently Galaxy S6 and S6 Edge users received Marshmallow beta update in UK by participating in this program. However, it is all up to Samsung to accept your registration or not. You must still give it a try.
So, to do this, you need ‘Galaxy Care’ app to be installed on your phone, which is now only available for UK users and specially for only two models, i.e. Galaxy S6 and S6 Edge with model number SM-G920F or SM-G925F. And these should be unlocked phones. There are also some more requirements which you can see in this Detailed Guide of how to install Marshmallow on these devices.
You can download the Galaxy Care app from Galaxy App store. Or, alternatively get the APK file directly from here and flash it manually. It is just 8MB in size and downloads in one click.