>>376457675
if you change anything about the model (especially the armature), the changes won't be reflected in your scene and you'll basically have to delete it and start over. just don't ever do that. you literally never need to unpack the prefab, for any reason.
even for the "lower arm is not the first child of upper arm" warning, you can just go to GameObject and set an object as first sibling. this works even if the prefab isn't unpacked.