Ok, so lets say I want to make the Draugr in Bloodmoon more muscular? Or just editing their body in general. How do you go about doing that???

You'll need to import the mesh files into some modelling program (Blender/3dsMax). There's a some tutorials for older versions of blender http://niftools.sourceforge.net/wiki/Blender. Apparently there's also beta versions of the same tools for new version of blender out there as well, but I haven't tried them out yet.

Which version should I get then?