Sony has officially announced what mobiles will be updated to Android 6.0 Marshmallow. If you are using any Xperia Smartphones and currently running Android Lollipop on it, check the list if your device is included for the update.
Android 6.0 Marshmallow update for Sony Xperia Smartphones
Xperia Z5 Compact
Xperia Z5 Premium
Xperia Z4 Tablet
Xperia Z3 Compact
Xperia Z3 Tablet Compact
Xperia Z2 Tablet
Xperia C5 Ultra
Xperia M4 Aqua
This time all the Xperia Z1 series smartphones have been excluded, and those users will keep running Android Lollipop on their devices.
Sony has not mentioned anything when they will start rolling out update. They only said the update will be released gradually, and some users/careers/devices may have delay receiving the updates.
Along with Sony some other smartphone manufacturers have also published the list of smartphones those will get the latest Android updates, you can read these articles depending on what device you are using.