Consider the basic steps that are required here:

1. Detect your collision
2. destroy object1
3. instantiate object2

Short of writing your script for you (which I try to avoid), that's your answer. Please remember to search through old answers before posting; this kind of thing get asked often and there are plenty of solutions there to look through.

make a gameobject variable and assign your second object into it and add the following script to your first object. it instantiates the second object when the first object collides with something.

var object2 : GameObject;

function OnCollisionEnter (col : Collision) {
     Instantiate(object2, transform.position, transform.rotation);
     Destroy(gameObject);

}