Every server has a set of acceptable default file names for each directory that get served when the user requests the root URL for that domain/directory. These are generally the file names you know, "index.html", "default.html", etc., but it can be set to a different name, if needed.
However, changing the default directory is an entirely different matter. It can be done in two different ways. (1) You can set the default directory on the server, or (2) you can set up redirects so that all requests for the root directory and redirected to the selected alternate directory. With method 1, the user never sees the directory name in his browser. With method 2, he does. This is strictly a matter of choice, but the details depend on the type of software that runs your server - usually Apache or Microsoft IIs. What software runs your server?