Sony Xperia C5 Ultra was launched in January 2015 with Android 5.0 Lollipop out of the box. When the Marshmallow was launched company says the device will get the marshmallow update. So now after long wait Sony is currently rolling out Android 6.0 Marshmallow on Xperia C5 ultra.
The update is rolling out on E5553 and E5506 variants with build number 29.2.A.0.122 and dual SIM models, E5533 and E5563 with 29.2.B.0.122. The update includes all the Marshmallow goodies like Doze and App permissions.
You can download the Android 6.0 Marshmallow firmware files for the Xperia C5 Ultra(registration required).