change textures to make facial expressions

I intend to use textures in the eyes and mouth to represent facial expressions just like xbox avatars. So, how can I replace some regions of the face with new textures?

Separate the polygons that form the face, UV map them separately, then you will use a second material for the face. Then all your facial expression textures and swap them (e.g. renderer.material.mainTexture = angryTexture)