Editing the source of a minimised javascript

I'm new to Javascript and web development, but I've been writing iOS software for the past 7 years. I'm not a complete noob. Here's the issue I'm having. I'm having a minimised Javascript file which is inserted in the website on the internet. How can edit the original source, while testing this on the website online where the bug in the Javascript is appearing. Google Chrome appears to have a function like that, but I can't seem to make it work. Any tips, resources or explanation so I can get this thing figured out?

Thanks.