Sony began tinkering with a cleaner build of Android 5.1 earlier this year with the help of a few hundred beta testers in Sweden. The so-called Sony Concept for Android program is moving forward today with an expansion to more markets and a bump up to Android 6.0 Marshmallow. Slots are limited (increased from last time), and it's not available in all regions. For once, it's the US that's getting left out.
The Marshmallow concept build of Android will only run on the Xperia Z3 and Z3 Compact. There are 10,000 spots available in the program, but only a handful of countries are included. You can try to sign up if you live in Sweden, Denmark, Norway, Finland, Iceland, Estonia, Latvia, Lithuania, United Kingdom, Ireland, and Germany. There's a Sony Concept app in the Play Store that you can install (only on eligible devices) to manage the process.
Spots are available on a first come, first serve basis. Sony will reach out to testers in the coming weeks with details on the next step. There will probably be plenty of rough edges in the concept software, but it's probably the only way you're going to see Marshmallow in the near future.