please can some one tell how to fix the mesh on zbrush
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I don’t know that it’s possible to fix that at this point. Although if this is only one side of your model you could try masking the other side and then using Tool>Deformation>Smart ReSym. You should apply the deformation at all subdivision levels, beginning with the lowest.
As for preventing that kind of thing, it would really help to know what you did to get to this point to begin with. 
thanks, i will try it, i let you what will be the outcome.
Jan