First, open and login to the the website hosting cPanel (control panel) in a web browser.
The "File Manager" link in the cPanel should be located at the top of the main cPanel page, but you can also use the search function to search for "File". Clicking on the "File Manager" link will open the files in a new browser window.
The primary domain's website is housed in the "public_html" folder .
Files for addon domains on the website hosting will be located in folders with the actual domain name, or whichever folder you set as the "document root" of the domain when you created the addon domain. (See HelpDesk article: Add Domain in cPanel)
In the file manager, you can open folders to view files, move or copy files, download, upload, rename, edit, compress, change permissions, delete files, and also create new files and folders.
*See details on the cPanel File Manager features here: https://docs.cpanel.net/cpanel/files/file-manager/