(C#) change texture to 2d prefabs randomly

Hi guys!
i need to change the texture to a smart group of prefabs; the textures that I want use are in assets/texture/terrain and the name of the texture are "rocce1","rocce2","rocce3"....etc
I want to set the texture on the prefabs after it is instantiated.

thanks!
Dousi96

 

here where to insert the c# script!
thanks