We're well past the point where any Android enthusiast would be expecting (or perhaps more accurately hoping) that his or her phone will be getting an update full of Marshmallowy goodness, perhaps delivered by some confectionary fairy. Alas, for owners of the Xperia C4 and Xperia C5 Ultra, it's a bag full of Lollipops instead. Both phones are now being updated by Sony to Android 5.1, a version of the OS which we'll remind you is now well over a year old.
Both phones began receiving their respective software late last night, at least according to the updated pages on Sony's support site. The Xperia C4 and C5 Ultra also come in dual-SIM hardware variants, both of which are receiving their 5.1 updates at thesame time. Check Settings>About phone>Software update to see if the servers have a package ready for you - Sony recommends Wi-Fi, but if you have space left on your data plan, it might download over the cellular network as well. If Sony's rollout system isn't fast enough, all four models can be updated manually via the official PC Companion tool.
Getting a year-old operating system is about the same as getting a year-old side of salmon, at least to keen Android fans, and it's cold comfort even if the software is free. However, according to Sony's commitment made late last year, the Xperia C4 and C6 Ultra are on the list of phones slated to be given Android 6.0 updates... sometime. Based on the timing of the latest updates, one might reasonably expect them around a January 2017, but we can hope for a little more haste.