A new firmware update should be heading to the Xperia M (C190X) and Xperia M dual (C200X) judging by the PTCRB website. Two new firmware builds have been certified, with the Xperia M seeing build number 15.4.A.1.9 and the Xperia M dual seeing 15.5.A.1.5.
For the Xperia M, this would move the build number from 15.4.A.0.23 to 15.4.A.1.9 and for the Xperia M dual it would move it from 15.5.A.0.18 to 15.5.A.1.5. It looks like this will be a reasonable update to the first Android 4.3 Jelly Bean firmware that started to roll in March 2014. We’ll let you know if/when it starts rolling.
Xperia M build number 15.4.A.1.9 certified for C1904, C1905 variants
Xperia M dual build number 15.5.A.1.5 certified for C2004, C2005 variants