Once you have opened up the HTML screen that you want to edit into the main edit area, click the content box. This opens a simple editor on the right that can be used to make edits to the text. You can write new text into this editor or copy-paste text from another source. Once the edits have been made, hit "Preview" to review the changes instantly. If you're happy with the changes, hit save.
The HTML screen can have a maximum height of 504px, which is best suited for most phone hardware (Apple, Android and Windows).