How would I change the type of tree when an action is being performed?

My project is about forest fires and I want to simulate them using Unity. I have two trees (type1 and type2). Type 1 is the regular healthy tree while Type 2 is the burnt looking tree. I have spawned a lot of Type1 trees across my terrain. When the fire catches on to a specific tree I want the tree to change to be type2 so you would see the tree change into a burnt one. How would I do this?