If you previously did use sitebuilder that option is no longer available, but you can edit files under the file manager found at "System info & files".
The filetree is a bit different, but if you have your site under the main domain you would find them by using the public_html folder that has an arrow on the icon.
For added domains (as well as the main if you prefer) you would look at the filetree on the left and navigate "domains" -> "mydomain.x10host.com" (or what you have) -> "public_html". Hovering the file will bring up an options menu