Today Sony announced Android 5.1 Lollipop update to its Xperia C5 Ultra smartphone. It has been unexpected because Sony has planned Android 6.0 Marshmallow but now they have released Android 5.1.
However, Android 5.1 comes to all Xperia C5 Ultra models including the single-SIM E5553 and E5506, as well as the dual-SIM E5533 and E5563 models. The new software build number is 29.1.A.0.87 for single sim version and 29.1.B.0.87 for dual sim version.
Update is rolling out gradually so you need to be patient & you should get Android 5.1 definitely. You can also check manually as well as for the update.