Sony Xperia C4 and C4 Dual owners expecting a direct update to Marshmallow skipping Android 5.1 may be in for a surprise as Sony is rolling out Android 5.1 Lollipop update to these devices. Both these devices at the moment were on Android 5.0 Lollipop and were on the Sony official list of Xperia devices getting the Android Marshmallow updates. The update comes with build number 27.2.A.0.155 for the Xperia C4 and build number 27.2.B.0.155 for the C4 Dual.
The Android 5.1 update is rolling out to five of the six C4/C4 Dual versions with only the E5343 being excluded for now. The update will reach you using the OTA (over-the-air) hopefully within the next few days.