So I am wanting to edit some of the body parts of a couple creatures. Golden Saint and Dremora to be precise. I have seen these other Dremora with different heads, etc. and read somewhere how to do it but do not know where it was. I am familiar with nifskope and blender but need some specific info on this. Do I edit both the .nif and the x.nif or only one? And can it be done with only nifskope? Some simple instructions would be very helpful. Thanks.

I would say your best bet would be to go ahead and do what you wish to change in Blender, and then make sure to export all three, (nif, xnif kf) using http://www.fliggerty.com/phpBB3/viewtopic.php?f=9&t=5733.

Edit: Make sure to import with the nif and not the xnif by the way.

If you want to edit a creature, make sure you're editing the x[creature].nif instead of the [creature].nif, for example, if you wanted to edit a dremora like you said, you'd edit the xdremora.nif instead of dremora.nif. The game uses the x.nifs to render creatures and as far as I can tell the other two meshes, creature.nif and creature.kf, are mostly for animation.