how to change the filedof view slowly

its changing but i want it to change slowly

function Update () {

if (Input.GetKey (“w”))
{
Camera.main.fieldOfView += 100*Time.deltaTime;
}
}

Well, the speed factor is right there in your script:

function Update () {
    if (Input.GetKey ("w")) {
        // See this '100'? --------\/
        Camera.main.fieldOfView += 100 * Time.deltaTime;
        //  That number determines how fast the FoV will change. Decrease it.
    }
}

Decrase it using a public variable:

var FoVChangeRate : float = 100;

Camera.main.fieldOfView += FoVChangeRate * Time.deltaTime;

Camera.main.fieldOfView = Mathf.Lerp(Camera.main.fieldOfView, 100, 1*Time.deltaTime);
this worked perfictly