The file containing the "quiz in web webpage format" is the HTML file. This is not immediately useful to you.
What you need is the original JQuiz file. This probably has a name that ends in ".jqz". Let's call this the JQZ file.
If you have the JQZ file, then you can open that with JQuiz, make your changes, and then regenerate the HTML file.
If you don't have the JQZ file, you will either have to recreate it, perhaps referring to the HTML file, or alternatively, if you just want to make a few small changes, e.g. change titles or fix spelling mistakes, you can open the HTML file with a text editor, and make the changes directly to the HTML file.