Your web browser downloads and stores files for future use in order to speed up loading on subsequent visits.
Sometimes, this "cache" can hold on to old outdated or broken files and so your browser may need to refresh it's cache by downloading updated content.
You can do this manually by following the advice below.
Remember that your cache is browser specific, so if you are having issues with one browser - try a different one.
Mac users can easily install Google Chrome if Safari is having any issues.