Changing a Entity's Texture

[Acies]

There is no need to unwrap a model which already has a good UV map. What you need to do is to locate the .dds file which applies to your model of choice. In this case I think it's the:
Amnesia - The Dark Descent\redist\static_objects\sanctumbase\sanctumbase01.dds

The part you would want to change is the pillar form located in the middle right of the picture. So I suggest copying the files (model.dae and texture.dds) and place them in a new folder with the same hierarchy.

If this doesn't work or sounds unclear I will test the process out or answer any questions.