Login to your cPanel and choose File Manager.
There are many folders and files related to your account, which are necessary for it to function correctly. Never delete any of these files and folders. If you want to delete something, and you are not sure if it is safe to delete, try renaming the file or folder, instead. This way your website will think the file is gone, but it can easily be restored (by renaming it back to the original name).
To upload to your primary domain's web site, always place your files inside the public_html folder.
To upload to your Addon or subdomain's web site, always place your files inside the document root folder (which should be inside public_html).
The regular Edit tool will show you the web code, and then you would have to know HTML and similar code to make changes. There is also a Code Edit tool which colors the code for easier reading.
There is yet one more editor tool called HTML Edit. This will give you a program similar to MSWord, as it allows you to type plain text, create line breaks, insert multiple blank spaces, and create images and links. When you save, the resulting page looks just like it did in the editor. In the hosting world, this is known as a WYSIWYG editor (what you see is what you get).