How to modify an extension from the Chrome Web Store?

I've seen this extension in the Chrome Web store, which shows the newest videos of a YouTube channel. I want to modify this extension in order to make it work with a different channel.

How can I get this done?

You've got two options:

1. Contact the extension's developer, and ask for assistance.
2. Edit the source code

I'm going to explain how you can modify an extension, and install it, in easy terms.

Getting the source and modifying it

1. Install the Chrome extension source viewer.
2. Go to the page in the Chrome Web Store of the extension you want to modify.
3. Click on the yellow CRX button, and choose Download (screenshot).
4. Extract the zip file.
5. Read the source code, and change what needs to be changed (in your particular case, I had quickly identified that you wanted to change edit config.js and change the "channel" property). Save the changes.

Using the modified version of the extension.

6. Visit the Chrome extension page (chrome://extensions/).
7. Enable Developer mode, by activating the switch in the upper-right corner.
8. Click on the "Load unpacked extension" button.
9. Select the folder of your extension (to know which folder is correct, check whether the folder contains a file called manifest.json).
10. Confirm. You're done.

Unless you've made a mistake in either of these steps (including the modification of the source code), the modified extension should work as intended.

Ethics

Modifying someone else's extension for personal use is not a big deal. However, do not distribute this extension without the original developer's consent. The original author has put lots of efforts in creating the extension, and might even have his/her contact details on it. If you distribute a (modified) copy of the extension, the original author cannot update the extension any more, so s/he cannot resolve bugs.
Even worse, if you turn the extension in a bad shape, users will complain to the original author, because their contact details are on it.