Every web browser stores a cache of the websites you visit (e.g. your new flash website) to reduce bandwidth usage and server load which allows you to load websites quicker. This is called the browser's cache. This is a great feature except when you're are editing a new website. You may need to clear your browser's cache or temporary internet files in order to see certain changes you have made to your design (e.g. updating your menu, changing your page layouts).
Click here for a great article on clearing the cache for several popular web browsers.