It is a little black Allan Key fix screw in the plastic support of the tail; also there is another shiny philips screw in the top, screwing on in the plastic itself to close the bloom (be careful, after two removes, the screw destroys the plastic turn itself, so probably you will need to replace it for a normal screw with nut as I did).. Then you can move in & out; but also you can turn !!! so check that the rudder blades are well aligned when they rotate (parallel to the bloom at 0º and 90º).

Good luck