Generally speaking, you wouldn't do that from your web panel at all. You usually create these files on your computer, and the upload them via FTP, or sFTP, onto the server in your website's directory. You can actualoly use the WebFTP (net2ftp) program available from the Manage Domains page of the control panel to edit your files, but I do not recommend this - it's just easier and better to get used to editing the files with other tools.
The information on the following DH WIki page might be a big help to you:
You can do that by creating and index.html file that has your "coming soon" message in it, and uploading that file to your web directory (using FTP or sFTP).