Even though the person who wrote the JavaScript that you are using should have tested it to make sure it actually works that doesn't mean that you don't need to test it yourself once you add it to your page. You may have not quite followed the instructions correctly and have something in the wrong place that will prevent the script from working correctly. Another possibility is that you already have another JavaScript in the same page and the two will not function correctly together without one or both scripts needing to be modified.
Testing JavaScript is really easy. Simply open your web browser and display the updated copy of your web page from your computer. Since JavaScript runs in the browser you don't need to upload the files to your web hosting before you can try it out.
If the script doesn't work and you can't see where you went wrong in following the instructions then try creating a really simple test web page with very little content and try adding the script into that. If all your test web page contains is a heading and a couple of paragraphs of text plus the script then you can quickly confirm whether the problem is likely to be a conflict with something else in the page. If you do get it working there then you could copy pieces of your actual page into the test one until you find out what piece of code stops it working. That will at least allow you to provide detailed information about the problem when you ask for help.
The other possibility when installing the script into your test page is that you will realise that you didn't do one of the steps quite right when adding the script to your regular page and you can then go back to your page and correct your mistake.