The Android 4.4.4 KitKat update with firmware build 18.3.1.C.0.21 recently started to roll for the Xperia M2. Now the same build number has been released for the Xperia M2 Aqua (D2403 and D2406).
The D2403 variant of the Xperia M2 Aqua has been released for a number of regions including Malaysia, Russia, Ukraine, Philippines, Saudi Arabia Switzerland, Thailand, and Middle East. The D2406 variant has been released for the generic US market.
DOWNLOAD FTF: Xperia M2 Aqua (D2403) 18.3.1.C.0.21 Generic Russia
Thanks Ben and Vinícius!