How can improve the symmetry if it has deformated a little bit(in other words the halfs of the head became different in the area of the nose)
Mask off half of the model you want to keep. Save a Morph Target so you can revert some of the detail afterwards. Then use the Smart ReSym in the Deformation pellet. That will copy over the detail of one side to the other. Use the Morph Brush if there are any glitches.
Thanks!