How to manipulate incoming packets ?

When i see example.com , he send a lots of packets to my machine that each one contain many things such as:

Code:

---

<script type="text/javascript" src="https://www.example.com/3fr43r45tff4gh3.js"></script>

---

But i want know that how we can manage & manipulate incoming packets to for example :

Code:

---

<script type="text/javascript" src="https://www.example.com/happy_new_year.js"></script>

---

Do you know anyway?

PS: I found Burp Suite that claim to packet editing but i don't know how its work ....

If you use Firefox, the Firebug extension might help. You can analyze the AJAX packets send to your browser and any other data as long as it is coming from the page your are viewing.