Question
I leave in Houston, Texas and weather freeze about 4 days.
My Lychee, all leaves turn brown but not drop(5 feet high).
The Guava, all leaves turn brown but all drop(12 feet high).
The Papaya, couple trees are soft but the other three are still hard from the top to the bottom tree(6 feet high).
I do cover them with a bed sheet but they still not survive this time. Do I need to prune them down and when is the best time.  

Answer
The Lychee and Guava should be left alone until new growth emerges in a month or so (if they are still alive), scratch the bark occasionaly in that time span to check for green underneath , which will indicate the tree "is" still alive. Once new growth emerges, you will then be free to trim/prune off the dead wood. The papaya is on its own, it will either survive or slowly decline, and I am inclined to think that it will not make it, for they are very, very cold sensitive and I myself have stopped growing them here in South Florida for that reason, there really isn't anything you can do for them once they are stunned by a frost or freeze. Nick