public IEnumerator AsyncSend()
{
    ...
    yield return www;
    ...
}

IEnumerator coroutine = AsyncSend();
StartCoroutine(coroutine);   //http://docs.unity3d.com/ScriptReference/MonoBehaviour.StartCoroutine.html

void Update()
{
    ...
    //timeout
    if (...)
    {
        StopCoroutine(coroutine);  //http://docs.unity3d.com/ScriptReference/MonoBehaviour.StopCoroutine.html
        www.Dispose();   //http://docs.unity3d.com/ScriptReference/WWW.Dispose.html
    }
    ...
}