BASIC STEPS :

 1.) Open the Blend file, activate sculpt mode and sculpt!  Don't
     worry about moving the eye or neck seams out of place accidentally,
     they're locked in place and cannot be moved.
 2.) When you're done, click the "more" arrow beside the shapekey bin,
     choose "Create New Shape From Mix" and note the name of the new
     shapekey which appears in the bin!
 3.) Next, delete the Basis shapeky FIRST, then your sculpting one,
     then delete the "New Shape From Mix" key you created LAST OF ALL
     (so it will become the permanent shape of your mesh.)
 4.) Export to FBX (format:7.4bin, scale:all/local/1.0, forw:-z up:y
     items:arm+mesh, paths:copy, armature prim:y, 2ndary:x,
     type:limbnode, leafbones:no)

The main heads you want to replace with your modified mesh are :
  characters\models\geralt\head\model\h_00_mg__geralt.w2mesh
  characters\models\geralt\head\model\h_00_mg__geralt_tattoo.w2mesh
  characters\models\geralt\head\model\h_01_mg__geralt.w2mesh
  characters\models\geralt\head\model\h_01_mg__geralt_tattoo.w2mesh
  characters\models\geralt\head\model\ep1_mark\h_00_mg__geralt_mark.w2mesh
  characters\models\geralt\head\model\ep1_mark\h_00_mg__geralt_mark_tattoo.w2mesh
  characters\models\geralt\head\model\ep1_mark\h_01_mg__geralt_mark.w2mesh
  characters\models\geralt\head\model\ep1_mark\h_01_mg__geralt_mark_tattoo.w2mesh

There are others used in the shaving scenes at Emperor Emyr's palace, but I don't
usually bother with those as it's not worth the effort for a few seconds worth of
screen-time (which only half of people even see.) :)

Since Geralt is always at the center of the screen, I don't bother with a LOD for his
head either, but instead remove the LOD directives from the material definitions for
each head (ie. Delete the non-zero "LOD_info" line that's usually on line 6, plus the
entire "material1" block which is usually lines 15-18.)  This will force the modified
high-res head to be used even in rare cut-scene instances when the camera is far away.

See here for a list of my actual mods... :p