The Xperia M (C1904, C1905) has become the first Sony handset to receive official CyanogenMod 13.0 nightly builds. This milestone is primarily down to the hard work of developer PecanCM (Ron Gokhale) over at XDA, who had been working on an unofficial CM13 beta since November 2015.
This means that Xperia M owners can now move to Android 6.0.1 Marshmallow through these official nightlies. The update is reasonably stable with most features working. If you are an Xperia M owner and want to give this a try, you will need to make sure you are running Android 4.3 Jelly Bean firmware and have an unlocked bootloader with a recovery installed (either CWM or TWRP).
If you are an Xperia M Dual owner, then unfortunately you won’t be getting official nightlies due to a known data bug that affects the second SIM. However, there are patches available to help enjoy this latest update (see the XDA source link below).