Even though Android 6.0 Marshmallow has been officially released last October 2015, not a lot of devices have it as of now, due to some delays with either the OEM or the carrier. But if you already have it, but feel like you want to play around with it more (and that you have the capability to do so), then you might want to try omniROM, a free software that’s based on Android, as they will be releasing nightly builds of Marshmallow soon enough.
If you’re not yet familiar with omniROM, it’s like the CyanogenMOD build, but some developers felt like the latter was becoming too commercialized, and so they created the former. Included in the initial nightly bulls (which started last January 10) are the Nexus devices, specifically 5, 5X, 6, 7 (2013), and 10. They will also be adding Nexus 4, Find 7/a, and OnePlus One in the next several days.
Users may also notice that previous Omni settings have been restored in this build, due to the time gap between the AOSP and Omni Marshmallow, as well as the changes that the new update has brought. This includes Status Bar Network Traffic Indicators, Silent Mode, unlinking notification and ringer volumes, separate lock screen wallpaper, etc. New features include weather in the notification header, enhanced Do Not Disturb mode, etc.
Google has been giving monthly security releases in light of recent malware attacks, and omniROM is aiming to also have these security patches applied as soon as the AOSP source is posted.