[SVGLead]

If I have a website that has a button and that button generates a random number through another website, how can I edit the response.
Button
I click the button, which sends a GET request to /randomNumberGenerator, which sends a response to my computer.
That response is intercepted by a program, and I want to be able to change that number to anything I want.
The website receives my edited packet and I have a "random" number on my website.

[flamery]

There are many tools to do this I will list a few:

Fiddler
Charles
Burp

[Lynx]

You can change the function they use to receive the number. Most likely they are using xmlhttprequest. Google this and see how the response is used. Good luck OP