Question about appearance.

I apologize in advance if this question has already been asked. It probably has, but I couldn't find it. Could you please tell me how the appearance of units can be changed in Heroes of Might and Magic V - Tribes of the East? I've downloaded some textures that have already been modified by someone else, and I liked them. But I would like to change a few more. How can I do that? What programs are needed for this? Thank you in advance.