Long ago I worked at a mental health center and a therapist there shared a technique for dealing with nightmares that had worked for some of his clients.  He reported one instance of a victim of a tractor rolling over him resulting in recurring nightmares of the experience.  In another instance a woman had recurring nightmares of learning two of her friends had been shot to death. My friend said that it was possible to change a nightmare while you were dreaming it.  The secret was to make the initial change very small and of little significance.  I subsequently attempted this procedure with a man who complained of a recurring nightmare.  Several sessions later I inquired how his nightmares were and he said he had just done what I suggested and they quit.