How to make code modification in the lib folder take effect in a JavaScript library project

I know this must be a very naive question, however, I can read code well but lack experience working on projects, and thus don’t quite understand the relationships of all the folders and files within a project.

So I want to make some custom modification to the Shaka player by changing code in the files in the lib folder. After changing it, however, I do not see the changes take effect. So I guess the files in the lib folder isn’t directly used by the code in the demo folder, and some rebuild or recompile should be done?

I’d appreciate some advice how to let my changes in files in the lib folder take effect?