How do I replace the index.html file.??

When a Web address doesn't end with a particular web page (often one that ends with ".htm" or ".html"), the browser looks for a file called "index.html." The error message you're getting means that there is no "index.html" file there. You need to create one and upload it. The details of how you upload a page depends on where you're trying to upload it and your relationship to that site. Likewise, the details of how your create an HTML page are beyond a brief answer.