you assign the NiSourceTexture by selecting the NiTexturingProperty you want to change then click the "+" in front of "Base Texture" and then change the "Source" in the ancestor ghost it will be "31 [NiSourceTexture]" you'll want to change it to the new NiSourceTexture #
Edit: Wow i didn't get Ninja-ed
This won't work as tronvillain has already stated that it changes the texture of the whle model.
NiTexturingProperty is where you normally put the texture. It is the parent of the NiSourceTexture, so don't worry, you were doing it right
but in this case the parts of the ancestor ghost share properties so doing this will change every texture in the mesh. Normally you can just use the split properties spell to allow you to assign different textures but that won't work on the Ancestor Ghost as all the NiTexturing properties point to the same branch. In this case you can just right click and copy the branch that you want to be seperated, paste it back in the same place and
then delete the old branch. Then you can change this texture individually. Make sure that you do the exact same thing in the x.nif of the ancestor ghost too otherwise it won't work in game.