You won't be able to restore the same site with the same database and directory location without first removing all your current files. There isn't any option in the plugin to do this type of restore.
Technically you would have to do the following:
1. Make a full backup of all your site files and Database. Files can be backed up via FTP to another location or some cpanels let you create a zip of a directory which you can FTP/move to another location. The database can be exported via phpMyAdmin (in your cPanel). This first step is just a pre-caution should you not be able to get the Duplicators installer to work correctly on your site. You don't have to take this step however I would strongly encourage it.
2. Login to your wp-admin and create a package from the Duplicator dashboard page. Then download the package and installer file to another safe location.
3. Remove all files in the current sites root directory.
4. Copy the package and installer back up to the empty directory root you originally had all your files in. This directory will obviously be empty as you just removed all the files.
5. In the web browser, browse out to the installer.php file and fill in all the parameters and re-install the site.
If this is the first time you have ever done this kind of work, then I would strongly encourage you to try it on your own computer by setting up a system like XAMP, WAMP, or for Mac MAMP.
Hope that helps!