I've not had a duck prolapse, only a chicken, but I suspect its much the same.
Clean the prolapse and put either some honey or Prep H cream on it (they both help to shrink the prolapse) and then gently push it back in. At first it will seem difficult but all of a sudden it will pop back into place.
You may have to do this several times a day at first but eventually it should stay put of its own accord.
There are videos of how to do it all on youtube
Good luck