Long awaited CyanogenMod 13 nightlies with latest Android Marshmallow is up and is coming to select devices.
Cyanogenmod said that the list will expand later.
They suggest if you are already unofficial build of the CM13 source code, developers suggest you to do a full wipe to come up to there Official nightlies, as they cannot guarantee upgrade compatibility.
If you are planning dirty flash from CM12.1 must update their third party add-ons like OpenGapps or Xposed immediately after flashing of CM13.
Of you want to get any more info visit to the Cyanogenmod site.