There are three steps to changing a building sprite:
- Import the sprite in to your project and make sure its configured as a Sprite in the Texture Import settings.
- Find the SpriteManager in your scene and add the sprite to the list of Building Sprites.
- Update the Building Configuration File (e.g. BuildingData.xml) that defines your BuildingType data and change the spriteName to match the name of your sprite.
A video of changing a sprite can be seen here: